Xnec2c: Graphical NEC2 Antenna Simulation

Introduction

Xnec2c is a GTK3-based Graphical version of nec2c, my translation to the C language of NEC2, the FORTRAN Numerical Electromagnetics Code commonly used for antenna simulation and analysis. The original nec2c is a non-interactive command-line application that reads standard NEC2 input files and produces an output file with data requested by "commands" in the input file. In contrast xnec2c is a GUI interactive application that (in its current form) reads NEC2 input files but presents output data in graphical form, e.g. as wire frame drawings of the radiation pattern or near E/H field, graphs of maximum gain, input impedance, VSWR etc against frequency and simple rendering of the antenna structure, including color code representation of currents or charge densities. These results are only calculated and drawn on user demand via menu items or buttons, e.g. xnec2c is interactive and does not execute NEC2 "commands" in batch style as the original does. Printing of results to an output file has been removed starting from v1.0, since xnec2c works in a way that does not allow printing compatible with the NEC2 format. If printing to file is needed then it is better to use the original NEC2 program, to avoid bugs that may still be lurking in the C translation.

Xnec2c now has a built-in editor for NEC2 input files which can be used to edit geometry or command "card" data. This basic editor displays comment, geometry and command cards in tree views where individual rows, each representing a card, can have their cells edited directly for "raw" entry of data. More useful are pop-up "editor" windows that open when appropriate buttons are clicked or when a selected row is right-clicked with the mouse. These editors allow easier, more convenient entry and editing of individual rows, with no need for detailed knowledge of "card" formats. When editing is completed, the contents of the nec2 editor can be saved in a NEC2-compatible input file which can then be re-loaded by xnec2c for execution.

Features

Multi-threading operation on SMP machines

Since v1.0, xnec2c can run multi-threaded (by forking) on SMP machines, when executing a frequency loop. Multi-threading is enabled by using the -j <n> option, where n is the number of processors in a SMP machine. xnec2c will spawn n child processes, to which it will delegate calculation of frequency-dependent data for each frequency step. Thus data related to n frequency steps will be calculated concurrently and passed on the the parent process by pipes, to be further processed for graphical display. Child processes are spawned before GTK is initialized and started so that only the parent process is tied to the GUI interface. Thus there are n+1 processes running when the -j option is used and execution is faster by slightly less than n times.

On-demand Calculation

Since xnec2c collects data to be displayed in buffers directly from the functions that produce them, there is no need to produce and parse an output file and no need to re-run the program when certain input data (currently the frequency) is changed or when different output data (gain, near-fields, input impedance etc) is required. The frequency can be changed either from spin buttons in the Main and Radiation Pattern windows or by clicking on the Frequency Data window's graph drawing area. The frequency corresponding to the pointer position will then be used to re-calculate whatever data is on display.

Built-in NEC2 input file editor

Xnec2c has a built-in editor for NEC2 input files. Data in NEC2 "cards" can be entered or edited either directly in the main editor window (tree view) or in more convenient dedicated editors for each type of card. Edited data can be saved to a NEC2 input file and reloaded for execution so that the edit-execute-display cycle is quicker and more convenient.

Accelerated Linear Algebra Support

Support for accelerated libraries was added in v4.3. Accelerated math libraries such as ATLAS, OpenBLAS and Intel MKL can speed up xnec2c EM simulations if available on your platform. Library detection details are available in the terminal. See File → Math Libraries → Help for more information. Accelerated operation is optional, it will fall back to the original NEC2 algorithms if necessary. Accelerated library support has been tested on Ubuntu , Debian, CentOS/RHEL, and VOID Linux. Generally speaking, if you can install the requisite libraries, it will be detected. If libraries are not detected on your OS then please open a bug report.

Interactive Operation

Xnec2c is interactive in its operation, e.g. when started it just shows its Main window in a "blank" state, indicating that no valid input data has been read in yet. The NEC2-type input file can be specified at start-up in the command line optionally with the -i option or it can be opened from the file selection dialog that appears via the File → Open menu of the Main window. Once a valid input file is opened, all the normal widgets in the Main window appear so as to allow proper operation. The NEC2 "commands" in the input file are read in but not executed, until a request is issued by the user via buttons or menus in the appropriate windows.

User Interface

Noise Temperature Visualization

The Radiation Pattern window can render each solid-angle cell of the pattern by its gain-weighted brightness temperature (K/sr) under a selectable RF noise environment, showing where noise enters the antenna. Eight environment models provide T_sky and T_earth values spanning quiet rural to dense urban conditions. An elevation control virtually rotates the antenna pattern to the desired observation elevation. The Frequency Plots window displays T_ant, T_total, and G/T_ant across the operating band.

Average Power Gain

When the RP card specifies at least two theta and two phi steps, xnec2c computes the average power gain over the radiation sphere after each frequency step. The result is printed to the console as a linear ratio, in dBi, and as a radiation efficiency percentage. An ideal lossless antenna with complete sphere coverage produces an average gain of 1.0 (0.0 dBi, 100% efficiency); lower values indicate resistive or ground losses. This diagnostic is enabled automatically unless --skip-verify is active.

Color Coding

Xnec2c uses color coding to visualize the Current or Charge distribution in the Structure's segments or patches as well as the Gain pattern or the Near E/H field pattern. Color coding is also used to clarify the Graphs of Frequency-related data. A color code strip is shown in the Main and Radiation Pattern windows.

Compilation and Installation

Binary packages are available via Flathub for Linux (thanks to AsciiWolf in issue #29), MacPorts for macOS (thanks to Carl Makin in issue #66), and FreeBSD FRESH ports. Xnec2c is known to build from source on many different platforms including Linux. If you have trouble building on your OS then please open a bug report.

To compile the package, it may be preferable to first run the included autogen.sh script in the package's top directory, to produce a fresh build environment. Then the configure script can be run with optional parameters to override the default settings and compiler flags, e.g: ./configure --prefix=/usr CFLAGS="-g -O2" will override the default /usr/local installation prefix and the -Wall -O2 compiler flags.

Running make in the package's top directory should produce the executable binary in src/. Running make install will install the binary into /usr/local/bin by default or under the specified prefix. It will also install the default configuration file into the user's home directory. This will have to be edited by the user as required. There is also this hypertext documentation file which you can copy to a location of your choice.

No configuration files are needed and the sample input files that were used during development are in the examples directory. Please also read the doc/nec2c.txt file that describes the nec2c application that is used as the basis for xnec2c.

Operation

Command Line Options

Usage: xnec2c [options] [<input-file-name>]

-i|--input <input-file-name>

Specify a NEC2 input file to be opened at start-up.

-j|--jobs <n>

Number of processors in SMP machine, where n is the number of processors (-j0 disables forking).

--openblas-threads <n>

Set OpenBLAS thread count (default: 1).

--mkl-threads <n>

Set Intel MKL thread count (default: 1).

--omp-threads <n>

Set OpenMP thread count (default: 1).

-b|--batch

Enable batch mode, exit after the frequency loop runs.

--optimize

Activate the optimizer immediately.

-P|--no-pthreads

Disable pthreads and use the GTK loop for debugging.

-h|--help

Print usage information and exit.

-V|--version

Print xnec2c version number and exit.

-v|--verbose

Increase verbosity, can be specified multiple times.

-d|--debug

Enable debug output (-dd includes backtraces).

-q|--quiet

Suppress debug/verbose output.

-c|--config <config-file-path>

Specify an existing config file to use (file must exist).

-C|--new-config <new-config-file-path>

Specify a new config file path (creates the file if missing).

--skip-verify

Skip geometry verification checks.

--force-verify

Force overlap check on large models (models with more than 1000 segments).

The following arguments write to an output file after the frequency loop completes. These are useful to combine with --batch; If you wish to specify filenames to write without --batch mode then enable the File → Optimizer Settings or the files you specify on the command line will not be written.

--write-csv <filename>

Write results in CSV format.

--write-s1p <filename>

Write S1P Touchstone file.

--write-s2p-max-gain <filename>

Write S2P file using maximum gain.

--write-s2p-viewer-gain <filename>

Write S2P file using viewer gain.

--write-rdpat <filename>

Write CSV file of radiation pattern.

--write-currents <filename>

Write CSV file of currents and charges.

--write-gnuplot-structure <filename>

Write gnuplot structure file.

--write-patch-currents <filename>

Write CSV file of surface patch currents.

Command Line Notes

<input-file-name>: Specify a NEC2 input file to be opened at start-up. If the -i option is omitted, xnec2c will take the last argument to be the input file path name, but will only open it if it has the .nec extension.

<number of child processes to spawn>: Since v1.0 xnec2c can run multi-threaded on SMP machines. This option specifies the number of child processes to spawn by forking, so that the total number of processes running will be n+1. n should be equal to the number of processors in a SMP machine.

Starting in v4.3, xnec2c always forks at least one job for better UI responsiveness. If you wish to disable forking for debug or testing then specify -j0.

Internationalization

Xnec2c supports 42 languages via GNU gettext. The language is selected automatically from the system locale. To override, set the LANGUAGE environment variable before launching — for example, LANGUAGE=de xnec2c for German. If the compiled-in locale directory is not correct for your installation, set XNEC2C_LOCALEDIR to point to the directory containing the LC_MESSAGES trees.

The Main Window

Once an input file is opened, the structure display can be rotated around the Z axis and tilted about a horizontal axis through the origin. This can be done either by pressing button #1 and dragging the structure with the mouse pointer, clicking the X (x), Y (y), Z (z), or Default (d) view button (45° rotation and tilt). The actual value of rotation and tilt is shown in two spin button widgets which can also be used to change the viewing angle.

Starting with v2.1, the structure display can be zoomed in or out by using the mouse wheel or the Zoom controls: Zoom % spin wheel and the Zoom In (Ctrl+Plus), Zoom Out (Ctrl+Minus), and Zoom 100% (Ctrl+1) buttons and it can also be moved around by dragging with the right mouse button #2.

The current distribution or charge density in the structure can be displayed by clicking the Currents (i) or Charges (v) toggle buttons at the top right of the Main window. The distribution of current or charges is rendered by a color code, red for the maximum value and magenta for zero. The Frequency Loop control buttons can be used to Start, Pause or Reset the loop. There is a Color Code bar at the left of the second row of widgets in the Main window, indicating the color coding and the maximum value of the displayed quantity (at its right).

The title in the border of the drawing area widget shows the user-selected function of the display, while the text entry widget at the right of the color code bar shows antenna gain in the Viewer direction, e.g. perpendicular to the Screen. To the right of this the Frequency Spin Button shows the current frequency in MHz for which the current/charge distribution and Viewer gain are calculated and displayed. If the Redo Check box is active, each time the frequency is changed in the spin button, all relevant data on display will be recalculated. If not, clicking the Redo button will initiate recalculation.

Printing of results to an output file has been removed starting from v1.0, since xnec2c works in a way that does not allow printing compatible with the NEC2 format. If printing to file is needed then it is better to use the original NEC2 program, to avoid bugs that may still be lurking in the C translation. Otherwise, it is possible to save the structure drawing to a PNG file by using the Save (s) or Save As (Ctrl+s) items in the File menu.

Starting with v2.1, xnec2c can save the structure display as a data file for the gnuplot plotting program. This is done by using the File → Save As gnuplot menu item, to open a file chooser dialog. If only the stem of the file name is given, xnec2c will automatically add the .gplot extension. Plotting in gnuplot is done with the splot <filename> with lines command, although the plot can be enhanced with some of the style etc commands available in gnuplot.

The View menu allows opening of other output data display windows and selection of various options: The Radiation Pattern menu (r) item opens the Radiation Pattern window so that the Gain pattern or the Near E/H fields can be calculated and displayed. The Frequency Data menu (f) item opens the Frequency Data Plots window which allows the plotting of various frequency-related data against the frequency range specified in the FR command. It also allows quick selection of the current frequency and recalculation of data by clicking on the plots drawing area. The Polarization submenu allows the selection of different polarizations for which many data items are calculated (e.g. gain, F/B ratio etc). The selection is global, e.g. it effects all relevant data that are drawn or displayed in other windows. The Common Projection item couples the projection (viewing angle) parameters of the Structure display in the Main window and the Gain or E/H field display in the Radiation pattern window so that both move in step. The Current Visualization submenu selects how current flow is depicted on the structure; see Current Flow Visualization. The Animate item opens the near-field animation dialog. The Symbol Overrides item opens the SY parameter tuning window. The OpenGL Settings… item opens the renderer configuration dialog.

The toolbar includes an Orthographic toggle button that switches between perspective and orthographic projection; see Orthographic Projection. The Rotate and Incline spin buttons accept mouse-wheel scrolling in 5° increments and have extended ranges (Rotate 0–720°, Incline ±360°) to allow continuous dragging past the original limits; values wrap automatically at 360° and ±180° respectively. With the OpenGL renderer active, Ctrl+Scroll adjusts the visual thickness of wire segment cylinders (equivalent to the Cylinder Scale slider). Scrolling down past the minimum snaps to line rendering mode, where segments draw as thin lines instead of 3D cylinders; scrolling back up returns to cylinder mode. This does not affect NEC2 model calculations.

The Radiation Pattern window

The View menu, in addition to the Common Projection check button, provides submenus for selecting Polarization type, Gain Scaling (which includes Noise Temperature modes), Noise Environment, Draw Style (Surface, Wireframe, or Both), a Gradient Key toggle for the color legend, and the Near Field data to be drawn. The selection of polarization type affects the Gain pattern, the displayed Viewer gain and the value of max and min gain as shown to the left and right of the color code bar. The selection of gain scaling only affects the form of the Gain pattern drawing: Linear Power is the most realistic, since the distance from the origin of each point in the gain pattern is proportional to the radiated power density, as is the color code (red for max gain and magenta for min gain). A disadvantage of this scaling is the inadequate representation of side lobes since they are usually significantly weaker than the main beam. Linear Voltage is better in this respect since the position of points in the gain pattern is proportional to Electric field strength and hence follows a square root law. ARRL Style follows a form of logarithmic scaling suggested by the American Radio Relay League, e.g. exp(0.058267 × gain) where gain is in dB₁₀. Finally Logarithmic follows a logarithmic scale with a median of 40 dB.

When a Near Field (NE or NH) command is included in the input file, clicking the E/H Field (f) button produces a drawing of the near Electric and/or Magnetic fields. By selecting the Near Field → Poynting Vector menu item the Poynting vector is also drawn. These fields are depicted by lines of fixed length in the direction of the relevant (E/H/Poynting) vector at each point in the drawing. The field strength is depicted by the color of the lines as using the line length for this purpose makes most lines too small to be useful. The drawing of the Near E or H Fields can be enabled or disabled by the Near Field → Near E Field and Near Field → Near H Field menu items.

The Near Field → Animate Dialog menu item (also available from View → Animate in the Main window) opens a dialog for setting animation parameters. The Frames/Sec spin button sets the target frame rate (1–120, default 30). The Animate Frequency spin button specifies a fictitious (slowed down) excitation frequency for which the animation is performed; the default is 1.0 Hz. Clicking Start begins the animation; Stop halts it; Close dismisses the dialog. Changing either spin button value auto-applies after a 500 ms pause, restarting the animation without pressing Start.

The Total Field sub-menu allows the selection of drawing either the Peak value or a "time-frozen" Instantaneous (φ=0) snapshot of the instantaneous value of the total Near Field E/H vectors. The Snapshot values are calculated as the vector sum of the X, Y, Z components of the E/H field and the Peak values are calculated using the formula NEC4 uses to print the Peak field values.

The Overlay Structure menu item enables the drawing of the structure in the radiation pattern drawing area when the Near Field pattern is selected for drawing. This makes it easier to understand the scale and extend of the Near Field patterns around the structure. The color scheme for the structure becomes white when Overlay is enabled, unless it is overridden by either the Current or Charge distribution being enabled by the relevant buttons in the Main window. With the OpenGL renderer, Shift+Scroll scales the overlay structure relative to the radiation pattern (gain view only; inactive in near-field mode), and Ctrl+Scroll adjusts wire cylinder thickness as in the Main window.

In the second row of widgets, the Color Code bar shows either the max and min values of Gain in the radiation pattern or the maximum value of the field strength in the near E/H field pattern. (Of course only one value can be shown, the precedence being E field, H field or Poynting field strength, depending on which of these is enabled in the View → Near Field sub menu). The Text Entry widget at the right of the color bar shows gain in the direction of the viewer (perpendicular to screen), while the Frequency Spin button following it shows the current frequency in MHz for which data is displayed. It can also be used to enter a new frequency in the same manner as in the Main window. The Redo Check box enables re-calculation and display of data when the frequency value changes, while the button to its right causes same when clicked by the user, but only if a new frequency has been entered.

The Gain pattern draw style is selectable from View → Draw Style: Surface renders a filled triangulated mesh, Wireframe draws colored line segments, and Both overlays wireframe on a dimmed surface. In all modes each element is colored according to the average value of gain associated with its vertices. The pattern can be "dragged" with the mouse pointer to rotate or tilt it and it can also be positioned using either the X (x), Y (y), Z (z), or Default (d) buttons. The Rotate and Incline spin buttons can also be used to accurately position the Gain pattern in the window. The label in the drawing area's frame gives information on what is on display and also the type of polarization or gain scaling.

Starting with version 2.1, the radiation pattern display can be zoomed in or out by using the mouse wheel or the Zoom controls (Zoom % spin wheel and the Zoom In (Ctrl+Plus), Zoom Out (Ctrl+Minus), and Zoom 100% (Ctrl+1) buttons) and it can also be moved around by dragging with the right mouse button #2.

Both the Gain and Near Field patterns can be saved as PNG image files by using the File → Save menu (s) or File → Save As menu (Ctrl+s) items. The Save option will save the drawings with a suitable default file name which includes a serial number, so that consecutive Saves do not overwrite files.

Starting with v2.1, xnec2c can save the radiation pattern and near E/H field display as a data file for the gnuplot plotting program. This is done by using the File → Save As gnuplot menu item, to open a file chooser dialog. If only the stem of the filename is given, xnec2c will automatically add the .gplot extension. Plotting in gnuplot is done with the splot <filename> with lines command, although the plot can be enhanced with some of the style etc commands available in gnuplot.

Noise Temperature Display

Two additional entries in the View → Gain Scaling submenu render the radiation pattern by noise temperature rather than gain magnitude. Noise Temperature colors each solid-angle cell by its gain-weighted brightness temperature in K/sr (Kelvin per steradian), using a linear scale. Cells receiving high brightness temperature from the environment appear at the red end of the color bar; those contributing little noise appear at the magenta end. Noise Temp (log scale) applies logarithmic compression, revealing low-level sidelobe contributions that the linear mapping compresses into a narrow color band.

The color legend adapts to show the K/sr range of the current pattern rather than dB gain values. Relative dB marks along the legend are suppressed, since they have no meaning in temperature units. The gain readout below the color bar likewise displays K/sr. To verify correct operation, observe that the highest-temperature cell corresponds to the direction facing the warmer hemisphere (earth, when elevation is 0° or positive) with the strongest pattern gain in that direction.

Noise temperature computation requires gain data over the full sphere. Xnec2c displays a one-time warning when it detects conditions that compromise the result. Two such conditions exist: a ground-plane model provides gain data for only the upper hemisphere, leaving the lower hemisphere—where earth noise would normally dominate—without computed values; and a beam maximum that lies significantly off the model's forward axis can distort the G/T_ant figure of merit, since the reference gain no longer represents the intended receive direction.

Noise Environment Models

The View → Noise Environment submenu selects which brightness temperatures are assigned to the sky and earth hemispheres. Eight models are available, drawn from three independent sources. Each defines T_sky and T_earth as functions of frequency.

The VE7BQH models use tabulated T_sky and T_earth values at discrete amateur band frequencies, interpolated logarithmically between points. These are the standard temperatures used in the widely-referenced VE7BQH antenna comparison tables, making VE7BQH Residential the natural default when comparing results against published data. For worst-case urban or contest scenarios, VE7BQH City or ITU-R Business represents the noisiest conditions. For remote EME or radio astronomy sites, ITU-R Quiet Rural represents the lowest terrestrial noise floor.

The ITU-R models derive T_earth from the man-made noise coefficients in ITU-R Recommendation P.372. At higher VHF frequencies in quieter environments, this formula can yield values below the physical ambient temperature; xnec2c clamps T_earth to a 290 K floor in these cases, since no terrestrial environment can be colder than its physical surroundings. To observe the clamping, select ITU-R Quiet Rural and increase frequency above 200 MHz while watching the reported T_earth.

The selected environment is persisted in the configuration file and restored on next startup.

Elevation Offset

The T Elev° spin button in the toolbar sets the observation elevation angle. The range is −90° to +90°. The antenna pattern is virtually rotated so that its maximum-gain direction points at the specified elevation above the horizontal plane. The sky/earth boundary remains fixed at the geometric horizon: cells facing upward after rotation see T_sky, cells facing downward see T_earth. In the 3D display the pattern visually tilts upward by the elevation amount while the ground stays at the bottom. Positive values model an antenna pointed above the horizon where more of the pattern sees cold sky; negative values point the antenna downward, increasing earth hemisphere coverage.

This setting affects both the 3D noise temperature coloring and the scalar T_ant computation. To confirm the relationship, set elevation to +90° and observe that T_ant approaches T_sky (the entire pattern faces sky). At −90° it approaches T_earth. At exactly 0°, an isotropic antenna produces T_ant = (T_sky + T_earth) / 2, since the two hemispheres subtend equal solid angle and every cell has identical gain.

The elevation value is persisted in the configuration file.

Understanding Antenna Temperature

Every physical object emits radio-frequency energy in proportion to its temperature. An antenna captures this emission through its radiation pattern; the total captured amount, expressed in Kelvin, is the antenna temperature—equivalent to the temperature of a resistor producing identical noise power at the receiver input. Xnec2c models the environment as two hemispheres (sky above the horizon, earth below), each carrying a brightness temperature set by the selected environment model. The RP command's angular step size partitions the pattern into solid-angle cells; each cell's gain determines how much brightness temperature it contributes, and summing over the full sphere yields the pattern temperature T_ant. Ohmic losses in the elements generate Nyquist noise that adds a loss temperature T_loss, giving T_total = T_ant + T_loss. T_ant quantifies geometric noise rejection (sidelobe and backlobe suppression); T_loss is determined by conductor material and cross-section. The figure of merit G/T_ant—forward gain divided by antenna temperature, in dB/K—captures both.

The Y-factor (T_earth / T_sky) determines how much leverage pattern optimization provides. Strong contrast between hot earth and cold sky means pattern shaping has a pronounced effect on captured noise. The environment models display T_sky and T_earth at the operating frequency; the ratio between them indicates how much G/T_ant improvement is available through pattern shaping at that frequency.

For further reading on antenna temperature evaluation, noise environment models, and G/T system analysis for VHF/UHF weak-signal design, see the Antenna Temperature and G/T reference by Hartmut Klüver, DG7YBN.

Frequency Data Plots window

The Frequency Data Plots window is the main display of frequency related data such as maximum gain, VSWR, input impedance etc. Most data can be plotted against frequency and some are displayed in text entry widgets. It is also a convenient way to quickly enter a new current frequency by clicking on the graph drawing area.

The following applies to all graphs plotted in this window: When a graph of two quantities against frequency is plotted (e.g. real and imaginary parts of input impedance), then one quantity is plotted in magenta color and its scale is at the left vertical side of the bounding box. The second quantity is plotted in cyan color and its scale is at the right side while a short descriptive title is printed in yellow at the top horizontal side. The graph bounding box is in white and the scale grid lines are in light gray. When only a single quantity is plotted against frequency, it is plotted in magenta color and the scale is at the left side of the bounding box.

Once graph plotting is complete (e.g. the frequency loop is done), clicking on the graph drawing area with button #1 (left mouse button) will produce a vertical green line in the graph bounding box, marking the new current frequency and triggering a re-calculation of all frequency-related data. Also, displays and drawings in all open windows (assuming the Redo check boxes are ticked active) will be refreshed to present the new data. Clicking on the drawing area with button 3 (right button) sets the frequency to the nearest frequency loop step value, as marked by the little boxes or diamonds on the graphs. However, all the displayed frequency-related data are still recalculated and refreshed e.g. buffered values are not used. Clicking with button 2 (middle button) cancels the green frequency-marking line.

The top row of widgets in this window has at its right buttons to select data to be plotted against frequency. These are:

The View → Net Gain menu item changes the second plotted quantity to the Net Gain of the array. Net Gain is the effective gain after subtracting the effects of reflection caused by impedance mismatch (return loss); Net Gain is equal to Raw Gain when VSWR ≈ 1.0 (S₁₁ ≈ −∞).

The Frequency Loop control buttons at the top right can be use to Start, Pause or Reset the loop. As the loop progresses, more data will be presented in the graphs and in the text entry widgets above the graph drawing area. These widgets display the current frequency in MHz, the maximum gain in the radiation pattern for that frequency, the VSWR for the Z₀ value in the spin button above, the real and imaginary parts of the input impedance, and—when noise temperature data is available—the T_total, T_ant, and G/T_ant readouts described in the Antenna Temperature and G/T Readouts section below.

The File → Save menu s and File → Save As menu Ctrl+s items can be used to save the graphs in the drawing area as PNG image files, with a default file name or one of the user's choice respectively. The View → Polarization submenu can be used to select the wave polarization type for which data is calculated and presented. When Viewer gain plotting is enabled, the graph will be re-drawn when the structure projection is changed by the various means described earlier (dragging by mouse pointer, Rotate/Incline spin buttons etc).

Starting with v2.1, xnec2c can save the frequency dependent functions as a data file for the gnuplot plotting program. This is done by using the File → Save As gnuplot menu item, to open a file chooser dialog. If only the stem of the filename is given, xnec2c will automatically add the .gplot extension. Plotting in gnuplot is done with the plot for [i=2:3] 'filename.gplot' using 1:i with lines smooth bezier title columnhead(i) command at the gnuplot console, although the plot can be enhanced with some of the style etc commands available in gnuplot. In this example it plots columns 2 and 3 (zreal and zimag) against column 1 (MHz). Change 2:3 to 2:16 to see all columns, though 16 plots will make the graph very busy.

Antenna Temperature and G/T Readouts

Three additional readout fields appear in the status bar when noise temperature data is available. T_total shows the total antenna temperature (pattern plus loss) in Kelvin. T_ant shows the pattern temperature alone. G/T_ant shows the receive figure of merit in dB/K. All three update as the frequency changes, whether by advancing the frequency loop or by clicking in the plot area.

The T_ant toggle button in the toolbar enables plotting of G/T_ant against frequency on the graph, using the right-axis scale. The View → Show T_ant instead of T_total menu item switches the secondary curve between T_ant and T_total, allowing direct comparison of pattern temperature versus total temperature across the operating band. Pattern temperature isolates the geometric contribution—how well the design rejects environmental noise through sidelobe suppression. Total temperature adds the ohmic loss component, determined by element material and construction quality.

G/T_ant versus frequency reveals the figure of merit across the band. When using VE7BQH Residential as the environment, values at the design frequency can be compared directly against published VE7BQH table entries. A rising T_ant toward band edges indicates growing sidelobe levels, since more of the pattern intercepts the warmer earth hemisphere. The gap between T_total and T_ant quantifies the ohmic loss contribution, which at 432 MHz and above can become significant relative to the pattern temperature depending on element material and cross-section.

Both display settings are persisted in the configuration file.

Touchstone Files

Touchstone files (or SnP files) are used by professional RF software packages like Microwave Office, Sonnet, and others. Now xnec2c can export .s1p and .s2p files to be used with those software packages. S₁₁ is the same as return loss, reflection and .s1p files provide single-port data. You can export Touchstone files from the Frequency Plots with File → Save as Touchstone (.SnP).

For .s2p files gain is used as S₂₁ and S₁₂: we assume the antenna is passive so S21 == S12. S₂₂ is a bit of a mystery, so we assume that all S₂₂ behavior is normalized into S₁₁ and thus S₂₂ is deminimus and set it to −100 dB. This may not be a correct assumption, so please provide a suggestion here if you know a better way.

The S₂₁ and S₁₂ values in the .s2p come in two types: Max Gain and Viewer Gain. Max gain is the maximum gain of the antenna, whereas, viewer gain is the current gain of the antenna pointing toward the viewport of xnec2c. For a directional antenna, pointing the main antenna lobe toward the xnec2c viewport would give the same (or nearly the same) values as max gain.

Once you have your .s2p or .s1p files you can design a matching circuit or other RF behavior in your favorite RF design software.

OpenGL Rendering

When built with OpenGL support, xnec2c uses a hardware-accelerated 3D renderer for both the Main window antenna structure display and the Radiation Pattern window. The renderer provides lit cylinder geometry for wire segments, triangulated patch surfaces, coordinate axes with labels, ground plane visualization, and smooth vertex normals on radiation pattern surfaces. If OpenGL initialization fails, xnec2c falls back to the Cairo software renderer automatically.

The OpenGL renderer is enabled by default when available. It can be toggled at runtime from View → OpenGL Settings… or by editing the Use OpenGL Renderer line in the configuration file. All OpenGL settings are persisted across sessions.

OpenGL Settings Dialog

The View → OpenGL Settings… menu item opens the settings dialog, organized into four sections.

Rendering

OpenGL Renderer — toggle hardware acceleration on or off. Constrained Rotation — restrict arcball rotation to a single axis at a time. Orthographic Projection — see below. MSAA Samples — multi-sample anti-aliasing at Off, 2×, 4× (default), 8×, or 16×. Higher values produce smoother edges at increased GPU cost.

Structure

Per-element Brightness (0=black, 1=full) and Transparency (0=opaque, 1=invisible) sliders for wire Segments and surface Patches.

Radiation Pattern

Brightness and Transparency sliders for the pattern Surface, Wireframe, and Near-field elements. A Draw Style radio group selects Surface, Wireframe, or Both.

Scene

Brightness and Transparency sliders for the Ground plane and coordinate Axes. Transparency Only on Click — when active, per-type transparency applies only while dragging; otherwise transparency is always visible. Cylinder Scale — visual thickness multiplier for wire segments. Below approximately 0.1 the display switches to line rendering mode, drawing segments as thin lines instead of 3D cylinders. Also adjustable with Ctrl+Scroll in either window. Does not affect NEC2 calculations.

The Reset to Defaults button restores all sliders, MSAA, draw style, and transparency settings to factory values. The renderer toggle and constrained rotation state are preserved across a reset.

Orthographic Projection

When the OpenGL renderer is active, a toggle button in both the Main and Radiation Pattern window toolbars switches between perspective and orthographic projection. The button icon changes between a perspective cube and a flat cube to indicate the active mode. Orthographic projection is also available as a checkbox in the OpenGL Settings dialog. This feature requires the OpenGL renderer; the toggle is disabled when using the Cairo software renderer.

In orthographic mode, parallel lines remain parallel regardless of distance, and object size does not change with depth. This is useful for precise angular measurements, verifying element alignment, and flat inspection of radiation patterns where foreshortening would distort apparent lobe symmetry.

Patch Current Visualization

The View → Current Visualization submenu in the Main window selects how current flow is depicted on surface patch elements (SM/SP cards) during animation. This feature requires the OpenGL renderer and is only visible when the model contains patch geometry. Five modes are available in two categories.

Animated modes update continuously while the structure display is open. The selected mode is persisted in the configuration file.

Radiation Pattern Draw Style

The draw style controls how the 3D radiation pattern is rendered. It is selectable from View → Draw Style in the Radiation Pattern window or from the OpenGL Settings dialog.

Surface

Filled triangulated mesh colored by gain. Provides the clearest view of pattern shape and gain distribution.

Wireframe

Colored line segments only. Allows seeing through the pattern to observe rear lobes and the structure overlay.

Both

Surface with wireframe overlay. The surface brightness is automatically dimmed so wireframe lines remain visible. This is the default.

The View → Gradient Key toggle controls visibility of the color legend bar that maps the gain color range. The draw style and gradient key state are persisted in the configuration file.

NEC2 Input File Editor

Xnec2c has a built-in NEC2 input file editor to make the edit/save/execute cycle easier and quicker. The main editor window opens from either the File → New menu n or File → Edit menu e items of the Main window. The File → New menu item opens the editor with some default rows ("cards") that amount to a free space vertical dipole which serves as a simple example. The File → Edit menu is used to edit a NEC2 input file that is already open in xnec2c.

The main NEC2 input file editor can be used to directly edit rows if desired and indeed this is the only method available for editing Comments. The editor though has several dedicated sub-editors for each of the type of card that is indicated in the Buttons above the Geometry and Commands Tree Views. The dedicated editor windows open when these buttons are clicked (to add a new row) or when a selected row is right-clicked by the user with the mouse.

Main Editor Description

The Main NEC2 Editor window is divided into three Tree View areas, one for editing Comments, one for editing Geometry and one for Control Commands. Each tree view has editable rows divided into cells that correspond to NEC2 input file's card columns e.g. Card Name (CM) - Comment Text or Card Name (GW) - Wire Data (I1 I2 F1 F2 F3 F4 F5 F6 F7) etc. The text of the first CM card is displayed in the title bar of the Main and Radiation Pattern windows, providing a quick identification of the loaded model. Each row can be edited by selecting it with a mouse click and then clicking on a cell. This requires detailed knowledge of the format of each of the NEC2 input file "cards" and so this method is only useful for editing comments.

The main editor is controlled by the top row of buttons: The Add (Ctrl+a) button inserts a new blank row in whatever tree view has been selected by a mouse click. The Remove (Ctrl+r) button deletes a row that was selected by a mouse click and the Clear (c) button deletes all rows in a selected tree view and clears it. The Save As (Ctrl+Shift+s) button opens a file selector dialog for saving the data in the Editor to a NEC2 input file. The Save (Ctrl+s) button writes data in the Editor to an already open input file. The Apply check button, when checked, signals xnec2c to reload the edited input file for execution. The Geometry and Commands tree views each have an Add Var (SY) button that creates a symbolic variable SY card for the selected row's numeric fields, converting literal values into named parameters. SY expressions can also be typed directly into the numeric fields of any card editor — for example, entering L0/2 in the Y2 field of a GW wire editor substitutes the half-length at evaluation time.

Note: In xnec2c versions earlier than v2.0-beta, due to the complex file opening process followed by NEC2 (many data sanity checks and initializations etc), reloading the input file resulted in all open windows (radiation pattern, frequency plots) to be closed. This was always an awkward situation and slowed down work in the NEC2 input file editor. As of xnec2c v2.0-beta, the user interface as well as a fair amount of code in xnec2c, have been modified so that as far as possible, when an edited NEC2 file is saved and reloaded, or another NEC2 file is opened, xnec2c will not close open Radiation Pattern or Frequency Plot windows and will not completely reset internally. This allows the user to edit a NEC2 file in the Editor window and, after saving, to be presented with the new calculations on the structure being modeled.

Finally the Revert (Ctrl+r) button reloads the last saved state of the editor from the input file, to reduce the effort needed to recover from a big mistake like clearing a tree view accidentally!

Sample Dedicated Geometry Editor Description

Wire Geometry Editor

To make things easier, the Wire editor has spin buttons to specify Length Taper and Diameter Taper separately to hide the need for calculating the actual beginning and end diameters. Also the "Segs % λ" spin button indicates the wire segment length as a percentage of smallest λ and can be use to set the needed number of segments for each wire to maintain a uniform relative segment length for all wires.

Three geometry Editors (wire, helix, arc) have a spin button to specify wire conductivity in S/m. When the spin button value is greater than zero, the Editor will enter an LD card in the Commands tree view to specify a type 5 (wire conductivity) loading. This will result in all segments with tag number equal to that in the Editor to be loaded with the specified resistivity.

All editors (except for the GE card) have the following buttons along the bottom of the window: "New" inserts a new blank row in the tree view after entering edited data into the current row. "Delete Card" removes the current row (card) and closes the editor window. "Apply" enters edited data into the current row. "OK" enters edited data into the current row and closes the editor window.

As of v3.9-beta, the GH "card" editor has a new appearance, since the Helix producing code has been edited to allow the creation of a spiral. Both right-hand/left-hand helices or spirals can be specified with the radio buttons in the top row of the GH editor dialog. In the bottom row, the radii that specify the shape of helices or spirals can be entered in the relevant spin buttons. These can be linked so that values entered can be propagated to the right, to make editing easier if all radii are the same. Right propagation is controlled by clicking on the "chain link" icons - if the icon displays a linked chain, right propagation is enabled. Otherwise if a broken link, right propagation is disabled.

Sample Dedicated Control Editor

Excitation Command Editor

Symbolic Variables (SY) and Live Parameter Tuning

Antenna design often begins with a target frequency and a set of proportions—element lengths expressed as fractions of wavelength, spacing ratios derived from empirical data or simulation refinement. The SY card transforms these relationships from scattered numeric literals into a coherent symbolic framework where changing one value ripples through the entire model. This 4nec2-compatible extension brings parametric modeling to xnec2c, enabling rapid exploration of design variations without manually editing dozens of coordinate values.

SY Card Syntax

Symbol definitions appear in either the geometry or command section of the input file. Each SY card declares one or more variables using the format:

SY name=value
SY name=expression
SY a=1, b=a*2, c=b+1

Comma separation allows multiple definitions on a single line, evaluated left-to-right so that later symbols can reference earlier ones. Symbol names are case-insensitive; FREQ, Freq, and freq all resolve to the same value.

Expressions support standard arithmetic operators (+, -, *, /, ^) with conventional precedence. Mathematical functions include trigonometric operations (SIN, COS, TAN, ATN—note that angles are in degrees), square root (SQR), exponential and logarithmic functions (EXP, LOG, LOG10), and utility functions (ABS, SGN, INT, FIX, MOD, MAX, MIN).

Predefined constants eliminate magic numbers from antenna descriptions:

PI

3.14159265358979...

MM, CM, IN, FT

Unit conversion factors to meters (1E-3, 1E-2, 2.54E-2, 0.3048)

PF, NF, UF

Capacitance multipliers (1E-12, 1E-9, 1E-6)

NH, UH

Inductance multipliers (1E-9, 1E-6)

AWG_0 through AWG_20

Wire radii in meters for American Wire Gauge sizes

The included example examples/2m_yagi_SY_parametric.nec demonstrates how symbols cascade through a model:

SY FREQ=146, C=299.792458, LAMBDA=C/FREQ
SY L_REF=0.492*LAMBDA, L_DRV=0.468*LAMBDA
SY X_DRV=0.193*LAMBDA, RADIUS=AWG_10
GW 1 25 0 L_REF/2 0 0 -L_REF/2 0 RADIUS
GW 2 25 X_DRV L_DRV/2 0 X_DRV -L_DRV/2 0 RADIUS
FR 0 21 0 0 FREQ-5 0.5

Changing FREQ recalculates wavelength, which propagates to element lengths and positions—the entire antenna scales to a new band through a single edit.

Symbol Overrides Window

Select View → Symbol Overrides to open the tuning interface. The window displays all defined symbols in two collapsible sections: Input Symbols contain direct assignments from SY cards, while Calculated Symbols derive their values from expressions referencing other symbols.

Each row presents:

• Symbol name and its current evaluated value
• Override entry with checkbox—when enabled, this value supersedes the original definition during simulation
• Min/Max bounds defining the slider range
• Slider for continuous adjustment within bounds
• Expression (for calculated symbols) showing the source formula

When an override activates, the original value displays with strikethrough styling, providing visual confirmation that the symbol now draws from the override rather than its definition. Bounds auto-expand if slider movement or direct entry exceeds current limits—drag the slider past the maximum and the ceiling rises to accommodate the new value.

Auto-Apply Parameter Tuning

The Auto Apply checkbox transforms the Symbol Overrides window into a real-time tuning console. With this mode active, each slider adjustment or value change triggers an automatic sequence: a 300-millisecond pause allows rapid successive adjustments to coalesce, then xnec2c saves the override state, reloads the model, and recalculates all frequency points. A spinner indicates when calculations are in progress; additional changes queue until the current computation completes.

This mechanism integrates seamlessly with the optimization workflow. External scripts can modify the companion .sy file directly—xnec2c monitors both .nec and .sy files for changes, reloading automatically when either updates. The override file format stores symbol state in a straightforward text structure:

# Symbol overrides file
FREQ: min_value=100 max_value=200 override_value=146 override_active=1
LAMBDA: min_value=1.0 max_value=4.0 override_value=2.05 override_active=0

Override persistence means tuning sessions survive application restarts. The .sy file loads before SY card processing begins, pre-populating bounds and override values so that subsequent symbol definitions merge with existing tuning state rather than replacing it. The opt_active field marks whether each variable is included in built-in optimizer runs.

Expressions in Card Editors

When SY symbols are defined, their expressions appear directly in geometry and command card fields. Right-click a card row (such as a GW wire definition) in the NEC2 editor treeview and select Edit, or double-click the row. The card editor dialog shows coordinate fields populated with expressions — X1, L1/2, -L1/2 — rather than raw numeric values. The editor preserves these expressions through the edit cycle: changing a value and clicking OK writes the expression back to the model.

Expression errors during editing are batched and shown in a single consolidated dialog rather than one popup per field.

Antenna Geometry Optimization

Xnec2c includes a built-in optimizer that adjusts SY symbolic variable values to improve antenna performance across the frequency sweep. The optimizer evaluates each candidate geometry by running a full NEC2 frequency sweep, computing a weighted fitness score from user-defined goals, and converging toward designs that minimize that score. This works directly with the Symbol Overrides window — variables marked for optimization are the search dimensions, and the override min/max bounds define the search space.

Three windows provide live feedback during optimization. Open all three before starting:

• View → Frequency Plots — watch VSWR and gain evolve across the band as the optimizer tests candidates
• View → Radiation Pattern — observe beam shape changes in real time
• The main Structure window — see geometry adjust as variables change

While the optimizer is running it controls the frequency sweep exclusively. The frequency spin buttons in both the Main and Radiation Pattern windows are disabled, and frequency plot traces turn dark green to indicate that the sweep is managed by the optimizer. Clicking in the plot area to select a frequency is also blocked during this time. All controls are restored when the optimizer finishes or is stopped.

For the historical external-optimizer approach (where a separate program modifies the .nec file while xnec2c monitors for changes), see External Optimizers below.

Quick Start: Optimizing an Example Antenna

This walkthrough uses examples/5el_yagi_SY_parametric.nec, which ships with a pre-configured .opt file containing fitness goals for VSWR, gain, and beam direction. The companion .sy file is created automatically the first time you interact with Symbol Overrides.

Understanding the Parametric Model

The antenna's shape emerges from a chain of symbolic relationships. Each variable controls one aspect of the physical structure:

R0, R1, R2, R3 — element length ratios

Each director derives its length from the preceding element multiplied by its R factor: L1=L0*R0, L2=L1*R1, and so on. Values of R below 1.0 enforce the forward tapering required of a Yagi — each director shorter than the one behind it. Without this constraint the optimizer could produce geometries where a forward director grows longer than the reflector, violating the fundamental operating principle of a Yagi-Uda array. The expressions that define the geometry must represent the intended physical outcome, because the optimizer will find and exploit any freedom the model leaves unconstrained.

S0, S1, S2, S3 — element spacing (wavelength fractions)

Spacing between adjacent elements as fractions of wavelength, accumulated from the preceding element: X1=X0+S0*LAMBDA, X2=X1+S1*LAMBDA, etc. Absolute positions would allow the optimizer to place a director behind the reflector or reverse the element order entirely — geometries that have no meaning as Yagi designs. Cumulative offsets prevent this while directly controlling the coupling between adjacent elements, which determines impedance matching, bandwidth, and gain distribution across the sweep.

X0 — reflector position

Fixed at 0, serving as the coordinate origin. Moving the entire antenna through space changes nothing about its electrical properties, so this variable is not checked for optimization.

L_REF_SCALE — reflector half-length scale factor

Scales the reflector: L0=0.5*LAMBDA*L_REF_SCALE. Because L0 is the root of the multiplicative length chain — every director length ultimately derives from it through successive R factors — adjusting this single value rescales the entire antenna proportionally. Left at 1.0 in this example, though it can be optimized when the reflector length needs independent adjustment relative to the wavelength-scaled default.

RADIUS — wire radius

Unchecked in this example. Wire radius affects impedance and bandwidth but is typically determined by available tubing stock rather than optimization. For designs where wire diameter is a significant fraction of segment length, see the EK card.

Tutorial Steps

1. Open examples/5el_yagi_SY_parametric.nec.
2. Enable the frequency sweep and open plot windows: select View → Frequency Plots, enable VSWR and Max Gain graphs, and click the triangular Play button. Also open View → Radiation Pattern.
3. Note the baseline performance: maximum gain is approximately 10.57 dBi, but worst-case VSWR across the band reaches 5.68. Even at the peak gain frequency, VSWR is nearly 3.0 — far above the 2.0 threshold where most transmitters begin folding back power.
4. Open View → Symbol Overrides.
5. Check the Opt checkbox for R0, R1, R2, R3, S0, S1, S2, S3 — these are the design variables the optimizer will adjust.
6. In the Optimization expander, the .opt file has pre-configured fitness goals: minimize VSWR (target 1, weight 5), maximize Max Gain (target 12, weight 10), and minimize Gain Dev +X (target 1, weight 10). The third goal constrains the direction of maximum gain to lie along the +X boom axis. Without it, the optimizer could produce geometries where gain is high but the beam points off-axis — a parasitic array resembling a dipole with reflector wires on either side, or a design that radiates strongly in elevation rather than toward the horizon. Directional constraint ensures the optimizer pursues high gain only in the intended forward direction.
7. Select Particle Swarm (PSO) and click Start Optimization. Watch all three windows update as the swarm explores the design space. The status bar reports progress: Pass N/M Iter Evals Fitness Best Stagnant Cache.
8. When PSO completes, switch to Simplex and click Start Optimization again. Simplex refines the best solution PSO discovered, descending precisely into the nearest minimum — a complementary strategy to the broad search that preceded it.
9. After Simplex completes: worst-case VSWR has dropped to approximately 1.19, with most of the band below 1.07, and maximum gain measures about 10.14 dBi. The peak gain decreased slightly from the unoptimized value because the optimizer found a geometry that distributes performance evenly across the band rather than concentrating it at a single frequency. The antenna is now usable from end to end. Optimization is stochastic — both PSO and Simplex involve random initial conditions — so exact results will vary between runs, but should fall in the same general range.

Iterative Refinement

The weight and exponent values for each fitness goal determine how the optimizer balances competing objectives. Increasing VSWR weight prioritizes impedance match quality at the expense of gain; increasing gain weight does the reverse. Higher exponents penalize large deviations more severely than small ones — an exponent of 2 squares the penalty, making a measurement twice as far from target four times as costly.

When an optimization run produces unexpected results — gain that seems too low, or VSWR that the optimizer ignores in favor of another metric — the Score column in the fitness goals grid reveals where the optimizer is spending its effort. A goal with a large score relative to the others dominates the total fitness and steers the search; goals with small scores have little influence regardless of how far their measurements sit from target. To shift the balance, increase the weight or exponent of an under-represented goal so its penalty grows large enough to compete, or reduce those values on an over-represented goal to loosen its grip on the search. The formula display at the bottom of the grid confirms the effect: re-run a pass after adjusting and compare the per-goal score breakdown to verify that the optimizer now distributes attention across all objectives as intended.

Subsequent passes of the same algorithm start from the current best values and often discover nearby improvements the first pass missed. When switching between algorithms, the key parameter to adjust is the search region: reduce PSO's Search size on later passes to concentrate the swarm near the current solution instead of scattering particles across the full range. For Simplex, the Sizes list already defines a sequence of decreasing perturbations — each pass automatically uses the next smaller entry.

Performance note: optimization runs faster when near-field calculations are disabled. Uncheck NH/NE output in the command cards or disable current display in the structure window. Near-field computation adds overhead to every frequency step and is unnecessary when only gain pattern and VSWR matter.

Fitness Goals and Measurement Types

Each optimization objective occupies one row in the fitness goals grid, defining what the optimizer should improve and how aggressively it should pursue that improvement. The total fitness score — the single number the optimizer minimizes — is the weighted sum of all enabled goals evaluated across the frequency sweep.

Use the + Add Metric button to append a new goal row, or the − button on any existing row to remove it. The formula display at the bottom shows the complete fitness calculation: F = W1*reduce((transform)^exp) + W2*... = total.

Goal Row Columns

Enable (checkbox)

Include or exclude this goal from the fitness calculation. Disabled goals remain configured but contribute nothing to the score.

Measurement (combo box)

The antenna parameter to evaluate. See the measurement reference table below.

Value (read-only)

The current measured result, updated live during optimization from the best candidate's frequency sweep.

Transform (direction combo box)

How the raw measurement converts to a penalty score:

• min score — score = (max(v − t, 0) / √(t² + 1))exp + τ / (1 + max(t − v, 0) / √(t² + 1)). Penalizes values above target; score approaches zero as value improves below target. A small tension term τ provides a residual gradient so the optimizer continues to improve met objectives. Appropriate for VSWR, angular deviation, S11 return loss, and any metric where smaller readings indicate better performance.
• max score — score = (max(t − v, 0) / √(t² + 1))exp + τ / (1 + max(v − t, 0) / √(t² + 1)). Penalizes values below target; score approaches zero as value improves above target. Works correctly for both positive and negative targets. Appropriate for gain, front-to-back ratio, G/T, and metrics where larger readings indicate improvement.
• ± target — score = |value − target|exp. Penalizes deviation from target; score is zero when value equals target. Appropriate for impedance, phase angle, beam pointing, and metrics that converge on a specific number rather than an extreme.

Target

The goal value for the selected measurement. Its meaning depends on the transform direction: for min score, the upper threshold (score approaches zero below this value); for max score, the lower threshold (score approaches zero above this value); for ± target, the center point of zero penalty. The √(t² + 1) normalization smoothly transitions between relative error for large targets and absolute error near target zero, with the crossover at |target| = 1.

Exp (exponent)

Applied to the transform result before reduction. Controls penalty steepness: an exponent of 1 produces linear scaling, 2 produces quadratic growth that penalizes large deviations disproportionately, and 0.5 compresses the range so that large and small deviations contribute more equally.

Reduce (reduction combo box)

How scores from individual frequency steps combine into a single value for this goal:

• sum — adds every frequency step's score; total grows with the number of steps, weighting broadband compliance heavily
• avg — averages across steps; normalizes for step count, balancing contribution across the band
• min — selects the lowest-penalty step. This isolates the single frequency where performance is strongest, which suits single-frequency or narrow-band designs where only the best operating point matters and the rest of the sweep serves as context rather than constraint
• max — selects the highest-penalty step, forcing the optimizer to improve the worst point in the band; effective for ensuring minimum performance everywhere
• mag — root of summed squared scores: sqrt(Σ score²); emphasizes large outliers without ignoring small ones
• diff — spread between largest and smallest scores; penalizes uneven performance across the band, pushing toward flat response

Weight

Multiplier applied after reduction. Determines relative importance when multiple goals compete — a goal with weight 10 has twice the influence of one with weight 5.

MHz lo / MHz hi

Optional frequency band filter. When set, only frequency steps within this range contribute to the goal's score. Empty values include all frequencies from the FR card. Filtering allows different objectives for different portions of the band — for example, strict VSWR within a 500 kHz operating segment while allowing relaxed gain tolerance across the full sweep.

Score (read-only)

Current penalty score for this objective, updated live.

Measurement Reference

Noise Optimization Workflow

T_ant, T_total, and G/T_ant depend on the observation elevation set in the T Elev° control and the selected noise environment. Set both to their intended operating values before starting an optimization pass that includes noise objectives.

A two-stage approach produces the best results. In the first pass, optimize for antenna geometry and pattern behavior—gain, VSWR, F/B ratio, beam direction—without noise objectives. This establishes a sound electrical design. In subsequent passes, add T_ant or G/T_ant objectives to refine noise performance at the intended elevation. Combining both stages in a single pass risks the optimizer distorting the pattern geometry to chase a noise figure that depends on an elevation angle it cannot control.

Algorithm Selection and Advanced Settings

The algorithm selector in the Optimization expander offers two choices. A ? button beside the selector opens context-sensitive help describing the currently selected algorithm's parameters.

Common Settings

Stagnation count (default: 30)

Iterations without improvement before the current pass ends.

Tolerance (default: 0.001)

Minimum improvement threshold — changes smaller than this are not counted as progress toward the stagnation limit.

Max iterations (default: 100)

Hard cap per pass regardless of improvement rate.

Simplex (Nelder-Mead)

The Simplex algorithm constructs a geometric shape (simplex) in the space of optimization variables and iteratively moves its vertices toward lower-fitness regions through reflection, expansion, and contraction operations. It excels at refining a solution that is already near a good region.

Sizes

Comma-separated initial simplex sizes for multi-pass operation (e.g., 0.09, 0.075, 0.05, 0.025). Each pass uses the next entry, producing progressively finer refinement. The number of entries determines the number of passes.

Min size (default: 1e-06)

Convergence threshold for simplex diameter. When the simplex contracts below this size, the pass ends — the algorithm has exhausted the region it can usefully explore.

Temperature (default: 0)

Simulated annealing temperature. At 0, the algorithm runs as pure Nelder-Mead, accepting only improvements. Positive values introduce random uphill moves proportional to the temperature, intended to escape shallow local minima. In practice this is difficult to tune effectively — PSO with an appropriate search size is generally a more reliable approach to escaping local minima, since the swarm naturally covers a wider region without requiring temperature calibration.

PSO (Particle Swarm Optimization)

PSO distributes a swarm of candidate solutions across the search space. Each particle tracks its own best-found position and is attracted toward both that personal best and the best position found by its neighborhood. The balance between exploration (inertia) and convergence (cognitive and social attraction) determines how quickly the swarm collapses toward a solution.

Particles (default: 0 = auto)

Swarm size. When set to 0, the count is calculated automatically from the number of optimization variables. Larger swarms explore more thoroughly at the cost of more function evaluations per iteration.

Neighbors (default: 0 = auto)

Neighborhood size for the social component. Each particle is influenced by the best position within its neighborhood rather than the global best, which preserves diversity and reduces premature convergence.

Inertia (default: 0.9)

Velocity decay factor. Values near 1.0 maintain momentum and favor exploration; lower values increase damping and favor convergence.

Cognitive (default: 0.5)

Attraction strength toward each particle's personal best position.

Social (default: 0.5)

Attraction strength toward the neighborhood best position.

Search size (default: 0.25)

Initial random spread for particle positions, expressed as a fraction of each variable's min-max range and applied as a ± offset from the starting point. A value of 0.25 places particles within 50% of the total range centered on the current values. If the resulting positions exceed a variable's bounds, they are clamped to the boundary. On subsequent passes, reducing this value concentrates the swarm near the current best solution rather than scattering across the full range.

Status Bar

During optimization the status bar displays: Pass 4/4 Iter 47 Evals 330 Fitness 19.9103 Best 19.9103 Stagnant 46 Cache 0/330 [Complete]

Optimizer Configuration Files (.opt)

The optimizer stores its configuration in a .opt file alongside the .nec file, using GKeyFile INI format. The file is auto-saved on every widget change and auto-loaded when opening a .nec file if a matching .opt file exists. See examples/5el_yagi_SY_parametric.opt for an annotated example of the format.

External Optimizers

As an alternative to the built-in optimizer, external programs can optimize antenna geometry through a file-monitoring feedback loop. Neoklis, 5B4AZ first described this approach:

Xnec2c monitors its .nec input file for changes and re-runs the frequency stepping loop which recalculates new data and prints to the .csv file. It is therefore possible to arrange the optimizer program to read the .csv data file, recalculate antenna parameters and save them to the .nec input file. xnec2c will then recalculate and save new frequency-dependent data to the .csv file.

If the optimizer program is arranged to monitor changes to the .csv file, then a continuous loop can be created in which new antenna parameters are calculated and saved to the .nec file, new frequency dependent data are calculated and saved to the .csv file and the loop repeated until the desired results (optimization) are obtained.

This mechanism was first implemented in Xnec2c v4.2. External optimizers remain useful for custom fitness functions, genetic algorithms, or specialized workflows not covered by the built-in optimizer. The following packages are available:

• Xnec2c-gao is an external optimizer that uses a genetic algorithm to find solutions in a multidimensional search space. It is written in Haskell by Maurizio Di Pietro and hosted on github.
• Xnec2c-optimize is written in Perl by Eric Wheeler and hosted on github.

Activating .NEC File Monitoring

Xnec2c will monitor the currently open .NEC file when you select File → Optimization Settings and select "Reload and write data on .NEC file changes". Additionally, you need to tell xnec2c where to write its output file. You can do that with one of the --write-* command-line options or by checking one of the boxes in File → Optimization Settings such as "After calculation, write <file>.csv". The checkbox "After calculation, write <file>-structure.gplot" exports a gnuplot-compatible 3D structure file after each evaluation. The checkbox "After calculation, write <file>-patch-currents.csv" exports surface patch current data in CSV format. All output settings are persisted in the configuration file.

Enable the frequency sweep if it is not already turned on:

1. From the main window: select View → Frequency Plots
2. From the Frequency Data Plots window: enable a graph such as VSWR, and configure whatever you would like to see during optimization.
3. Click the triangular "Play" button to run a frequency sweep. This is a persistent setting that will auto-sweep each time xnec2c loads and it must be active in order for the optimizer loop to function.

Frequency loop timing information is available on the console when you increase verbosity with -v. Specify -v multiple times to increase verbosity. For example, with -vv using examples/2m_yagi.nec:

[info] 140.000000 MHz: 0.045850 seconds. (ATLAS, Threaded)
[info] 141.500000 MHz: 0.054378 seconds. (ATLAS, Threaded)
[info] 140.000000 MHz: 0.063772 seconds. (ATLAS, Threaded)
[info] 140.500000 MHz: 0.066472 seconds. (ATLAS, Threaded)
[info] 141.000000 MHz: 0.078861 seconds. (ATLAS, Threaded)
[info] 142.000000 MHz: 0.047910 seconds. (ATLAS, Threaded)
[info] 143.000000 MHz: 0.068066 seconds. (ATLAS, Threaded)
[info] 143.500000 MHz: 0.076449 seconds. (ATLAS, Threaded)
[info] 144.000000 MHz: 0.068083 seconds. (ATLAS, Threaded)
[info] 142.500000 MHz: 0.113185 seconds. (ATLAS, Threaded)
[info] 146.000000 MHz: 0.050178 seconds. (ATLAS, Threaded)
[info] 144.500000 MHz: 0.051486 seconds. (ATLAS, Threaded)
[info] 145.000000 MHz: 0.058897 seconds. (ATLAS, Threaded)
[info] 145.500000 MHz: 0.060019 seconds. (ATLAS, Threaded)
[info] 147.000000 MHz: 0.057313 seconds. (ATLAS, Threaded)
[info] 146.500000 MHz: 0.070968 seconds. (ATLAS, Threaded)
[info] 148.000000 MHz: 0.071760 seconds. (ATLAS, Threaded)
[info] 147.500000 MHz: 0.080575 seconds. (ATLAS, Threaded)
[info] 148.500000 MHz: 0.051870 seconds. (ATLAS, Threaded)
[info] 149.500000 MHz: 0.058890 seconds. (ATLAS, Threaded)
[info] 150.000000 MHz: 0.056449 seconds. (ATLAS, Threaded)
[info] 149.000000 MHz: 0.071011 seconds. (ATLAS, Threaded)
[notice] Frequency loop elapsed time: 0.467847 seconds. (ATLAS, Threaded)

Input File Considerations

Since xnec2c is interactive, it will not initiate calculations without a prompt from the user. For this reason certain NEC2 commands that normally cause execution (e.g. RP, XQ etc) are read in but not acted upon. Any data in the lines of these commands are saved for use when the user requests output data calculation and display, via buttons and menu items in the GUI. Also, since xnec2c was designed to visualize output data graphically, certain types of output data requests are not supported (e.g. the surface wave pattern option (I1 = 1) of the RP command etc). An error message dialog will hopefully appear to inform the user of unsupported commands or options.

Since xnec2c is an implementation of NEC2. You can learn more about the NEC2 format at https://www.nec2.org. Be sure to review Part III: NEC User's Guide. Here is a list of commands or command options not supported by xnec2c:

GF

Read Numerical Green's Function: Relevant code has been removed in nec2c since this type of solution is obsolete.

WG

Write Numerical Green's Function: Relevant code has been removed in nec2c since this type of solution is obsolete.

NX

Next Structure Data: Relevant code has been removed since xnec2c cannot operate in batch mode.

PQ, PT, CP

These commands affect printed output and have no effect on data presented by xnec2c in graphical form. Since v1.0 xnec2c does not print results to file.

SOMNEC

The separate SOMNEC code has been incorporated in nec2c and hence in xnec2c also.

EK

Activates the extended thin-wire kernel. The standard kernel treats each wire segment as an infinitely thin current filament — current flows along a line at the wire axis and the boundary condition is enforced approximately at the surface. The extended kernel incorporates the wire's finite radius into the field evaluation, accounting for the cylindrical geometry when computing interactions between nearby segments. This produces more accurate results when the segment length-to-radius ratio is small: the standard kernel requires a ratio of at least 8 for 1% field accuracy (minimum usable: 2), while the extended kernel achieves 1% accuracy at a ratio of 2 (minimum usable: 0.5). The additional computation per segment is negligible on modern hardware, so enabling EK is generally recommended. Xnec2c reports accuracy warnings when geometry violates the active kernel's validity conditions.

EX

When Incident Wave or Elementary Current Source Excitation is specified, xnec2c can only calculate and render the re-radiated field, produced by the current induced onto the structure. Only the initial values of the θ and φ angles are used and no stepping of these angles is performed. Therefore it is better to specify only one step for θ and φ in the EX card.

RP

The surface wave option (I1 = 1) is not supported.

SY

Symbolic variable definition for parametric modeling, compatible with 4nec2. Defines named constants and expressions that substitute into geometry and command card fields. See SY Symbolic Variables and Parameter Tuning for full documentation.

ZO

As of v3.7-beta, this is a new "card" addition to the input file, which I have incorporated to allow the Transmission Line Z₀ to be specified in the NEC2 antenna description file.

Output File Considerations

Printing of results to an output file has been removed starting from v1.0, since xnec2c works in a way that does not allow printing compatible with the NEC2 format. If printing to file is needed then it is better to use the original NEC2 program, to avoid bugs that may still be lurking in the C translation.

Keyboard Shortcuts

The following tables summarize all keyboard accelerators. Single-key shortcuts are active when the corresponding window has focus. Modifier combinations work regardless of widget focus within the window.

Main Window

Radiation Pattern Window

Frequency Data Window

NEC2 Editor

Version History (newest first)

Version 4.4.18

This release focuses on mathlib enhancements, platform compatibility improvements for MacOS and Fedora, and crash prevention through input validation.

• Contributors:
  • Carl Makin - MacOS testing, reported #73, tested MacOS build (#66)
  • likecabezon - reported #70
  • Richard Shaw - reported Fedora OpenMP build issue
• Features:
  • Thread count control via CLI options (--openblas-threads, --mkl-threads, --omp-threads)
  • MacOS MacPorts OpenBLAS support
  • Mathlib ID-based configuration for stable config persistence across version upgrades
• Bug Fixes:
  • Fix mathlib benchmark crash without loaded NEC2 file (#73)
  • Fix OpenMP linking in Fedora RPM builds
  • Fix strict-prototypes warning on macOS builds (#66)
  • Fix nearfield animation crash with missing NE/NH cards (#70)
• Documentation:
  • Thread control options documented in man page and HTML documentation
• Build system:
  • Fix CFLAGS handling in spec file to preserve OpenMP flags

Version 4.4.17

This release includes radiation pattern color legend enhancements, critical bug fixes for locale handling and CSV output, stability improvements, and build system updates.

• Contributors:
  • Ian - color legend suggestions, reported #61
  • Neoklis Kyriazis - technical guidance on current scaling (#61)
  • Devin Bayer - fixed double-free crash in geometry editor (#67)
  • hamarituc - reported #59
  • ProtonTheFox - reported #44, #45
  • likecabezon - reported #68
  • Jerry - reported #52
  • polprog - reported #50
  • roberthadow - reported #47
• Features:
  • Graduated color legend with relative/absolute gain marks (-3dB, -6dB, -10dB intervals)
  • Color legend with dynamic spacing and proper gain scaling
  • Increased 3D zoom limit to 999999x
  • GTK menu tear-offs for frequently used menus
  • "Begin Frequency Loop at Startup" config checkbox (#45)
  • Smooth pan/tilt/zoom during optimization
  • Responsive mobile support for HTML documentation
• Bug Fixes:
  • Fix NEC file format to use decimal points regardless of locale (#59)
  • Fix CSV output scaling for currents/charges magnitude (#61)
  • Fix double-free crash when editing geometry (#67)
  • Prevent deadlocks in Notice(), Open_Input_File(), and dialog scenarios
  • Fix endless loop with overlapping segments
  • Fix gain value display and color mapping
  • Improve error messages for segment tag lookup failures (#68, #47)
  • Fix custom frequency step in color legend
  • Fix radiation pattern color legend to match gain scaling styles
  • Prevent GTK beeps (#50)
  • Fix batch mode auto-start (#44)
  • Graceful child process exit handling
  • Reduce severity of math library detection failures to warnings
• Documentation:
  • Add nec2.org user manual reference (#52)
  • Update Ubuntu/Debian package suggestions (#46)
  • Modernize HTML documentation with improved styling
• Build system:
  • Add VSCode integration with debug configs and auto-configure
  • Generate c_cpp_properties.json from configure
  • Parallel build tasks support

Version 4.4.16

• Bug Fixes:
  • Fixed geometry calculations when reading NEC2 decks that use the GM card with tag=0 to copy all segments
  • Fixed segment lookup failures when tag=0 is specified
  • Improved error message to properly identify which GM card caused lookup failures
  • Improved radiation pattern color legend spacing
  • Fixed building on sparc64
• Code cleanups and improvements
• Contributors:
  • Diane Bruce - inspired code cleanup and helped with pull requests
  • Dave Hibberd - Debian maintainer who helped troubleshoot and provide guidance in solving the sparc64 build issue

Version 4.4.15

This release fixes a crash related to a GTK icon problem. Thanks to AsciiWolf for helping report and troubleshoot the issue.

Version 4.4.14

This release fixes a hang when the frequency plots window is too small for the number of plots being rendered. Thanks to Tom who reported the issue.

Version 4.4.13

This is a large bug-fix release and includes a few minor features and documentation updates. I would like to thank the following who helped with new features and bug fixes, in order of contribution:

• Contributors:
  • Neoklis Kyriazis
  • Rafal Lukawiecki
  • ndim
  • anoduck
  • rklasen
  • Ward
  • ChrisMaple
• Features:
  • Save window positions when they are closed
  • Tile rdpattern and freqplot windows on first run
  • Default the Radiation Pattern "Gain Pattern" to be enabled
  • Officially support OpenBSD, thanks anoduck for testing
  • Add save radiation pattern to CSV
  • Add save currents and charges to CSV
• Documentation:
  • Added External Optimizers section
  • Added Google Translate javascript for the website
• Bug Fixes:
  • Fixed deadlock when the frequency loop is running and a window is
  closed. Thanks to Neoklis Kyriazis for discovering the problem and proposing a solution.
  • Fixed edge case where structure window does not redraw on save. Thanks
  to Neoklis Kyriazis for discovering the problem and providing a fix.
  • Fix blank plots, structures and radiation pattern when Quit is pressed.
  • Open the Frequency Plots window when Optimization is enabled
  • Auto-run freq loop when optimization is enabled
  • Differentiate between windows at 0,0 and never opened
  • Correct a relatively trivial warning from Coverity Scan about an
  unused value
  • Fix warning from Coverity Scan about invalid argument to a function
  • Fix a warning by Coverity Scan about an unterminated buffer
  • Fix Coverity Scan warning about a tainted value used in malloc()
  • Fix a Coverity Scan report about a "Forward NULL"
  • Convert double-locking synchronization to use a condition
  variable. Thanks to anoduck for testing and reporting the issue in the process of adding OpenBSD support.
  • Fix warning: while loop has empty body
  • Fix real/imag swap
  • rdpat csv: reset φ on each frequency iteration
  • fix integer overflow in frequency selection, thanks to rklasen.
  • Fix sommerfeld ground excitement issue. Thanks to Ward for
  reporting the bug and writing a fix.
  • Fix off-by-one array index
  • Fix sprintf truncation when plotting. Thanks to ChrisMaple for
  reporting this issue.
  • Delay clicked frequency calculation while a freq loop is running
  • Fix beeps on GTK field overflow
• Build system:
  • Huge autoconf and buildsystem refactor by ndim
  • Updated autoconf to detect Mac OS X properly
  • Fix Debian Lintian desktop-mime-but-no-exec-code
  • Removed more LMID references
  • Changed host_os is not supported to be a warning
  • Actually check if backtrace exists.
  • Add g_module_supported detection
  • Added OARS metadata into AppStream
  • Updated autoconf to detect Mac OS X and set LDFLAGS accordingly. Big thanks to Rafal Lukawiecki for helping troubleshoot and get xnec2c building on OSX.

Version 4.4.12

This release has been pending for a while, there are quite a few changes. I thought about making this 4.5 but not yet since these are mostly maintenance items, with a few new features as well:

• UI Features:
  • Added File → Save-as CSV in the Frequency Plots window
  • Added optimizer output config checkboxes for csv, s1p, and/or s2p. Now you can auto-generate .csv, .s1p, or .s2p files when the input .nec file is modified by an external program.
  • Pre-processed CSV data for each example .nec file and placed them in examples/data/. You can view these with any spreadsheet program and graph columns as you like; the data is useful for regression testing, too.
  • Updated optimization documentation.
• Console Features:
  • Refactored console output based on output priority levels. Console output is now color coded and output level can be controlled with -v and -d options. See --help for details.
  • Add batch mode (-b) commandline flag.
  • Added commandline options --write-csv, --write-s1p, --write-s2p-max-gain, and --write-s2p-viewer-gain for use with --batch and the optimizer.
  • Added --optimize to activate the optimizer thread immediately.
  • Updated the man page
• Bugfixes:
  • Fixed octal representation of 0755 when creating ~/.xnec2c/
  • Added directory permission hotfix for users affected by 4e62893b
  • Do not invalidate current data when radiation pattern is enabled; Thanks to Gian Luigi for reporting this bug and providing .nec file samples to reproduce the problem.
  • Check if radiation pattern was calculated before using the RP data, Thanks to Gian Luigi for reporting this bug, too.
  • Added fallback PATH_MAX if undefined, like for GNU/Hurd.
  • optimizer: prevent loading zero-byte files
  • Disable optimizer if inotify is missing, this should fix xnec2c builds on BSD.
  • Fixed printf size_t warnings on 32-bit architectures
  • Fix type errors with glib 2.73 and later, Thanks to the Debian maintainer Christoph Berg, for his work to troubleshot and test this!
  • Fixed segfault when mathlib_idx or mathlib_batch_idx is out of range. Thanks to Christoph Berg for reporting this.
  • Only write optimizer data if FREQ_LOOP_DONE
  • Updated xnec2c.spec to reference the right dependencies in el8.
• Cleanup:
  • Renamed files with a : in them so checkouts will work in Windows.
  • Updated frequency calculation timing output.
  • mathlib: fixed duplicate BLAS detection, removed LMID support, simplified console output
  • Several spelling fixes, thanks to Daniele Forsi!
  • Fix cross compilation issues, Thanks Michal Vasilek!
  • Updated quickstart documentation
  • Removed LMID checks from configure.ac; again, Thanks Michal Vasilek!
  • Re-ordered change history in this document so the newest versions are on top

Version 4.4.11

There are many small improvements in this update, please update! Here is a brief summary, see the git commit log for detail:

• Updated icons and file associations so you can double-click to open .nec files in most desktop environments. Just make desktop-install after make install to set it up! Distribution maintainers: see the PACKAGING file for details.
• Added .so.0 filenames names for OpenBLAS detection in CentOS when the -devel package is not installed.
• Added the debian/upstream/metadata file as recommended by Debian for package info.
• Added xnec2c.spec for RPM-based builds! Just rpmbuild -bb xnec2c.spec to create an RPM. You will need to place the latest .tar.gz file in ~/rpmbuild/SOURCES or run spectool -g -R xnec2c.spec to download them for you. Thanks to Richard Shaw for reviewing the .spec file!
• Added include of locale.h to fix building under musl-libc. Thanks to Tom for pointing this out and testing the fix!

Version 4.4.10

Fixed crash caused memory allocation off-by-one error. Thanks to Juan Pablo García for reporting the bug!

Version 4.4.9

Minor fixes, both reported and tested by Juan Pablo García. Thanks for your help!

• Fixed S₁₁ phase angle calculation for graphs and touchstone file export.
• Fixed locale issues for numeric output data (touchstone, gnuplot, csv) and for xnec2c.conf when switching from one locale with period for decimal point to another with comma for decimal point, or vice-versa.

Version 4.4.8

This is a minor maintenance release.

• Fixed buffer overflow in the temp string used in mem_write_format(), thanks to Juan Pablo García for reporting the bug while testing touchstone file output.
• Changed default Save-As filenames to resemble the original .nec filename where appropriate.
• Avoid EM calculation if the selected frequency step (when you right-click on a graph) has already been calculated. Note that, at the moment, it must always calculate if left-clicking the graph or when "Show Currents" or "Show Charges" is selected.

Version 4.4.7

This release has a few minor features added:

• Touchstone 1- and 2-port files (.s1p, .s2p) can be exported from the frequency plots window. Touchstone files are used by professional RF software packages like Microwave Office, Sonnet, and others. Now xnec2c can export .s1p and .s2p files to be used with those software packages. S₁₁ is the same as return loss, reflection and .s1p files provide single-port data. Thanks to Juan Pablo García for suggesting this feature.
• Converted frequency plot gnuplot output to include all columns. Now all columns are available in the .gnuplot file. Thanks to scaramacai on github for the initial work on this conversion.
• Added auto-detection of musl-libc so it will compile for users with musl-libc. Thanks to Tom at Gentoo for suggesting and testing this change.
• See the github changelog for the lovely details.

Version 4.4.6

This is mostly a release with a handful of bugfixes and some internal re-organization in preparation for s1p/s2p output and gnuplot extensions (src/measurements.c). Detailed descriptions are available in the git log.

• Removed C++ compiler dependency, xnec2c is written in C
• Fixed default mathlib handling when no accelerated library is installed
• Fixed error on first run when ~/.xnec2c/ does not exist
• Added libtool dependency to documentation for CentOS
• Fixed segment fault when HOME environment variable is undefined.
• Fix off-by-one error in memory allocation for frequency plot arrays.
• Fixed deadlocks when running without pthreads and without forking.
• Refactored common measurements into src/measurements.c
• Refactored src/plot_freqdata.c to use meas_calc()
• Added function use comments to src/measurements.c
• Suppress max_freq == min_freq when freq_steps == 1
• Fixed gcc warning about the return value of getcwd() not being checked.
• Ignore the GN card if both GN and GE cards indicate freespace.
• Fixed format-truncation compiler warning

Version 4.4.5

• Corrected Im(-z) plotting on the Smith chart, it was plotting upside-down. Thanks to Juan Pablo García for pointing this out.
• Updated examples with corrections from Neoklis related to ground configuration.
• Updated GCC/Clang compiler optimization settings for better compatibility.

See the git commits for detail if you're interested!

Version 4.4.4

Fixed regression introduced in v4.3 by adding -ffinite-math-only as a gcc optimization. This causes radiation patterns to draw incorrectly when the RP card (radiation pattern card) when using a "Perfect Ground" in the GN card at the same time that "Radial wire screen" is used as the ground in the RP card. Thank you Neoklis for investigating the examples that produced the issue.

Version 4.4.3

Added option to Round X-axis Frequency Values:

X-Axis frequency values in the Frequency Data Plots window default to scaling based on the total frequency range and number of points which deviates from the way Neoklis originally implemented the plot. This makes the best use of graph space, however, this new form can create axis values that do not align nicely to whole numbers or common fractions of numbers (like 13.5 vs 13.72).

The new option View → Round X Axis Frequency Values will align the X Axis labels to normalized values as originally designed in releases prior to v4.1.5. When this option is off, it will show one more frequency significant figure displayed on the X Axis and scale the plot to use the entire plot space.

Version 4.4.2

• Fixed plot width drawing with multiple FR cards. This fixes a regression that had disabled mouse-wheel scrolling to resize plots with multiple FR cards.
• Fixed Frequency_Loop race conditions causing access to uninitialized data that were seen as flickering while running the frequency loop, or as incomplete plot drawings.
• Added strerror on failed file open for m-anish on github to troubleshoot error opening xnec2c.conf.
• Refactored memory handling routines to bypass realloc() if possible. This increases performance slightly, reduces heap thrashing, increases stability by clearing memory on reallocation, and makes memory handling easier to debug.

Version 4.4.1

Fixed graph scaling so that each point is drawn in order while running the frequency loop without rescaling the graph on the x-axis. This way the points line up correctly to their frequencies throughout the plot while the frequency loop is running instead of only being correct after all points have been plotted. Thank you Neoklis for pointing this out!

Version 4.4

• The File → Gain Scaling setting is now saved between restarts since this is a visual preference per user.
• GTK resource refactor prevents the need to delete the old resources/xnec2c.glade file on new releases that have UI changes. This is a pretty major change that should be transparent to users, so please report any regressions.
• Fixed a minor fsync() bug
• Updated installation details that will help distribution packagers. Special thanks to Tom DL1JBE at Gentoo for his packaging feedback!
• Added a PACKAGING file to help distribution packagers.

Version 4.3.5

Updated documentation around autogen.sh and added tests to make sure gettext and related tools are available for autoconf to complete successfully.

Version 4.3.4

• Fixed slow sync() issue by replacing it with fsync(). Thanks to bbarenblat on github.
• Fixed typos and spelling errors throughout the application, thanks to Daniele Forsi on github.
• Updated documentation and added official website links

Version 4.3.3

A few optimizer cleanups and bug fixes including updating CSV format to remove extra whitespace and increase precision of the NEC2 data. See git log for details.

Version 4.3.2

Additional optimizer trigger and inotify cleanup. Also xnec2c writes the .csv file as soon as the checkbox is selected to trigger any external optimizers waiting for the .csv to be modified.

Version 4.3.1

• Added an option to show S₁₁ (dB) (negative return loss) as a plot next to VSWR. See Frequency Plots → View → Show S₁₁
• Added an option to clamp VSWR to a max of 10 for graphing purposes. In the past xnec2c has always clamped to 10 so this option defaults enabled. If you want to see the entire VSWR curve no matter how big VSWR is, then see Frequency Plots → View → Clamp VSWR to 10.
• Fixed deadlock while optimizing when inotify receives a .nec change before xnec2c is done writing the .nec.csv file.

Version 4.3

• Changed maintainer to Eric Wheeler
• Moved xnec2c to github: https://github.com/KJ7LNW/xnec2c
• Major refactor of the plotting code:
  • Added multi-plot support when multiple FR cards are used.
  • Added View → Show Min/Max values to the Frequency Data Plots graph.
  • Enable click+drag for continuous frequency selection.
• Performance improvements:
  • 30% speedup of native NEC2 code Gaussian Elimination code with -fcx-fortran-rules
  • Added support for ATLAS, LAPACK, OpenBLAS and Intel MKL accelerated linear algebra libraries. These are auto-detected at runtime on CentOS/RHEL, Ubuntu/Debian, and openSUSE.
  • Added benchmark and time reporting for math libraries
  • Separated interactive vs batch math library to get the best of parallel graphing performance as well as interactive single-frequency performance.
  • Prevent calculating frequency dependent data twice when the program loads a new file.
  • Removed synchronous GTK+ loops to increase responsiveness.
  • Made GTK more responsive by converting Frequency_Loop() to run as a pthread
  • Made xnec2c always fork a child process for handling frequency data calculations requested by Frequency_Loop even for -j1 to prevent blocking GTK during a calculation (disable forking with -j0)
• Bug-fixes:
  • Fixed parallel job child communication races and better EINTR handling
  • Fixed multiple crashes when inotify detects a change of the .nec file while the frequency loop is running.
  • Fixed uninitialized data program-wide by calling memset(0) in mem_alloc().
  • Fixed Clang compiler support and added auto-detection
  • Fixed multiple multi-threaded fixes and optimization flicker problems
  • Prevent resetting the projection angle and zoom during optimization
  • Retry the optimization request after 100ms if there are busy child processes.

Version 4.2

Further to the changes above, I have modified xnec2c so that more than one FR (frequency range) card can be specified in the NEC2 file. This makes it possible to calculate frequency-dependent parameters over separate ranges of interest and thus reduces the time taken by the program to produce results. Unnecessary calculations between ranges of interest are also avoided.

Version 4.1.5

After an inquiry by Eric Wheeler regarding the use of the GH card to create spirals, I found a bug that caused the GH editor to delete the GH entry in NEC2 Editor's treeview. I also found a bug that caused the number of turns of a spiral to be rounded to the nearest integer. Both bugs have been fixed in this version.

Further, to aid Eric Wheeler to develop his Simplex automatic Optimizer, I added code to monitor, with inotify, the .nec input file and to trigger recalculation and plotting of frequency dependent data, while at the same time printing to file results (VSWR, max gain, F/B ratio etc). Also added code so that if the user clicks on the Frequency Plots drawing area, the underlying frequency is saved and re-entered into the frequency-dependent calculations. This, together with maintaining the viewing angle of the Radiation pattern makes it easier to monitor the Optimizer's progress.

Version 4.1.4

Added the stdint.h include file to src/common.h to fix a compilation error reported by Nick.

Version 4.1.3

Arranged the pipe reading function for the parent process to re-read from the child processes if not all data is transferred at the first read(). In this case xnec2c was crushing with "Resource temporarily unavailable" error message.

Version 4.1.2

Changed the installation commands in Makefile.am so that all the relevant files (files/xnec2c.desktop, application pixmap, configuration file, executable binary etc) are installed under any location specified to the configure script by the --prefix= option. Also modified the program so that on first start up after installation, the application will create its working directory by copying files from the relevant directories under the installation prefix.

Version 4.1.1

I made a small change to the Radiation Pattern code to allow the drawing of both horizontal an vertical "cuts" in the radiation pattern, in the zx and zy vertical planes and the xy horizontal plane. Also changed the accelerator keys for the Add, Remove, Clear, Save As, Save and Revert buttons to be Modified by the Control button as the original accelerator keys were being activated when writing in the Comments Treeview.

Version 4.1

I corrected some mistakes in the GUI interface code, which handles the creation of windows and dialogs from the glade UI description file. These mistakes were such that they did not effect the operation of xnec2c but in any case I fixed them for the sake of correct programming. Also changed the wire designer dialog to include the Tapered Wire selection check-button in the dialog's frame and the relevant code to deal with this better.

Version 4.0

I corrected some mistakes in the GUI interface code, which handles the creation of windows and dialogs from the glade UI description file. These mistakes were such that they did not effect the operation of xnec2c but in any case I fixed them for the sake of correct programming. Also changed the wire designer dialog to include the Tapered Wire selection check-button in the dialog's frame and the relevant code to deal with this better.

Version 4.0

I have changed the code that saves the state of the View-Polarization menu items, as it would only correctly save the state of these items in the Main (structure) window. I also changed the code that handles the removal of "card" entries in the NEC2 editor, as it was causing crashes of xnec2c under some user action combinations.

Version 3.9

Since I have not received any bug reports for some time after releasing v3.9-b, and having fixed a bug that resulted in a gtk_widget_destroy() command to be passed on an invalid widget pointer, I am releasing xnec2c as v3.9.

Version 3.9-beta

I have arranged for some user actions to be "nested", mainly regarding the saving of NEC2 editor data followed by the opening of another file or starting a new project. Some such functionality already existed in previous versions but it is now more comprehensive. Another useful change is fixing a bug in the original NEC2 code for producing a helix (as specified by the GH "card"). This bug resulted a division by zero if the helix length was specified as zero, possibly intended to produce a spiral. The GH card can now be used to produce both a helix or a spiral with no floating point exceptions.

Version 3.8-beta

I edited some of the code so that when a NEC2 file is saved, the Center x and y offsets (the change of position) of the structure and radiation pattern are not changed. I also arranged for the offset values to be saved in ~/.config/xnec2c.config when closing xnec2c, and to be read back and restored when starting.

Version 3.7.1-beta

Fixed a reported bug that prevented wire diameter entries less than 0.01 in the Helix (GH) card editor. This was due to specifying a minimum value of 0.01 in the resources/xnec2c.glade file.

Version 3.7-beta

After some feature requests by users of xnec2c, I implemented a number of changes to xnec2c so that it can save the state of the GUI at exit and restore it on start up. To this effect xnec2c now produces a configuration file xnec2c.config in the user's ~/.config directory. In this file xnec2c saves the state (position, size, active toggle buttons, projection parameters for structure and radiation pattern displays, the state of the frequency loop etc) of the main windows, e.g. structure, radiation pattern, frequency plots and NEC2 editor. I have also added keyboard accelerators for a number of buttons and menu items, more commonly used in the main windows of xnec2c, and a new ZO NEC2 "card" to specify transmission line impedance.

Version 3.6.1-beta

After a request by Don Walters I modified the resources/xnec2c.glade file to reduce the default size of the drawing areas as they would not fit in displays with reduced resolution. I also removed some legacy code, left over from the migration of xnec2c to GTK3, which was producing error messages from GTK3 during the dragging of structure or radiation pattern displays. And I modified the Makefile.am file to get make install to install program files and documentation to the right places.

Version 3.6-beta

I migrated Xnec2c to the latest GTK+-3.22 toolkit to bring it up to date. A lot of changes were needed to both the GUI and parts of the Xnec2c source code, mainly the use of GTK3's GUI Builder for creating the User Interface and the revised drawing model of GDK for presenting the Graphics displays. A new autogen.sh script has also been incorporated in the source package and internationalization has been migrated to the GNU GetText system. Because Xnec2c is very complicated and thorough testing is difficult, I am releasing this version as a beta.

Version 3.5.1

After a request by Fan Jun, I replaced the original makeshift desktop icon with one provided by Serge.

Version 3.5

I modified the NEC2 Editor code so that when a Geometry or Command editor window is opened, activating Apply or OK will save data in the editor window to the treeview, even if the default data is not edited.

I modified the NEC2 Editor code so that if a treeview row is removed while the relevant editor window is open, then activating Apply or OK will not attempt to save data to the treeview, since this will cause an illegal memory access and will crash xnec2c.

Fixed a bug in the Helix editor which caused incorrect calculation of the segment length as a percentage of λ or of the number of segments/turn that correspond to a given segment length as a percentage of λ.

Version 3.4

I fixed a bug in the code that implements the modified GS (scaling) card. In NEC2 the GS card scales all of the structure, but in Xnec2c I modified the code so that it is possible to specify a range of tag numbers to apply scaling to only. When a GS card follows a GX card, selective scaling doesn't work correctly because a symmetric structure is created. If you are using a GS card, it may be better to avoid specifying a tag number range, just in case the bug fix is not complete.

Version 3.3

I fixed a bug that was in the code that reads the commands from the NEC2 input file, where xnec2c would crash with a segmentation fault when the FR card was after the RP card. Also modified the Strtod() function to avoid freeing the end pointer before it was used, causing problems.

Version 3.2

I made some changes to the Strlcat() function and its usage in the xnec2c code, to improve safe handling of string concatenation operations. Hopefully this has not broken the handling of various strings in xnec2c!

Version 3.1

I have made several changes to the code that renders wire frame drawings and displays some data, like the projection parameters (azimuth and elevation of the structure and the radiation pattern) and gain values. This has resulted in a faster and smoother animation of these drawings and reduced workload on the processor during dragging of these images. I have also reduced the default minimum size of xnec2c's windows, as I had reports that they would not fit in the displays of some laptops or monitors, with resolutions on the lower side.

Version 3.0

I have modified the NEC2 file parser so that it can read the file with both the '.' and ',' as decimal point characters. This is necessary to make xnec2c compatible with different locales. I also fixed a resource (memory) leak, reported by valgrind, in the xyz axes drawing routine.

Version 2.9

After a request by the Debian maintainer of xnec2c, I added a rudimentary man page he sent me and corrected some spelling errors (arbitrary to arbitrary).

Version 2.8

Fixed a bug in the GN card editor function which caused xnec2c to save the GN card parameters in the .nec antenna description file without clearing the number of radials to zero. This resulted in the GN card editor window to open with confusing and incorrect defaults for the radial screen, when the Reflection Coefficient Approximation method was selected for specifying ground parameters.

Also fixed some bugs (missing variable initializations) in the Ground Parameters calculation functions which resulted in incorrect Radiation Pattern calculations. This would happen if a .nec file, with Perfect Ground (iperf = 1) specified in the GN card, was opened after a .nec file, with a Radial Ground Screen specified in the GN card, was processed.

Version 2.7

After a bug report by Tim, I fixed a bug that was preventing the creation of an LD card of type 5 (LDTYP=5) when wire conductivity (S/m) was specified in the GW card (Wire) editor. The same bug was present in the GH (Helix) card editor and it was also fixed. I added code so that the wire conductivity for GW, GA and GH cards can be read from the relevant LD card and shown in the Conductivity (S/m) spin button.

After all these changes I checked the xnec2c source code using the Coverity Scan service and I fixed an out-of-bounds read error that was reported by the scanner, in the src/plot_freqdata.c file.

Version 2.6

After a bug report by Lucjan, I changed the case of the signal handler that deals with SIGCHLD so that it doesn't cause xnec2c to exit when the SIGCHLD signal does not originate from child processes created by xnec2c. It appears that in some Linux installations a SIGCHLD signal is sent to xnec2c even if it is not running forked, e.g. the -j option is not used in the command line.

Version 2.5

After a bug report by Jean Collin, I made some changes to the input file parser code in src/input.c, to properly identify tabs in the input file.

Version 2.4

I submitted xnec2c (and nec2c) for scanning to the Coverity source code audit website, which produced a list of no less than 57 issues to be fixed! Most of them were not bugs that affected calculations but possibly two, both in the Somnec code, one in function rom1() and one in gshank() likely could have caused errors in the relevant calculations. Unfortunately the FORTRAN source of Somnec (as well as that of the NEC2) is very difficult to read so I cannot say with certainty if this was so.

Version 2.3-beta

Fixed a bug in the cell_edited_callback() function that caused segmentation fault crashes when a cell in the input file editor was edited directly by the user.

Version 2.2-beta

I made extensive modifications to the source code to silence a large number of warnings generated by the LLVM clang compiler when used with the -Weverything option. These were mostly cases of implicit conversions between variable types, like int to char or uint to int etc. I have also updated the basic files of the GNU Autotools build system, to be compatible with the current version of these tools at the time of writing (February 2013).

Version 2.1

Patched a problem in the graph plotting routines that caused xnec2c to crash with a division by zero fault. This can happen when e.g. a single-wire structure is specified and a plot of gain vs frequency is requested, for a polarization that is at 90° to the wire, e.g. requesting gain plot for horizontal polarization for a vertical wire. Since there is no radiation in the horizontal plane of a vertical wire, the values of gain given to the plotting routines are all the same so the vertical scale drawing routines crash xnec2c.

Version 2.1-beta

I have introduced many changes in this version, so I am releasing it again as a public beta version for testing and bug reports:

After a bug report by David J. Singer, I changed all declarations of variables that are used in memory allocations, from int to size_t. This error was in the nec2c code from the beginning but apparently it only showed up when extremely large memory allocations are requested in nec2c and xnec2c.

I have replaced all the (deprecated) GDK drawing primitives with equivalent Cairo graphics equivalents (e.g. replaced gdk_draw_line() with cairo_line_to()) since Cairo provides for nicer anti-aliased drawing.

After a feature request by David J. Singer and friend Richard, I have added code to save data of the structure display, radiation patterns and frequency plots into file, in a format suitable for the gnuplot plotting program.

Version 2.0-beta

I received a bug report from David Binderman regarding an array bounds violation, which he found by compiling xnec2c with the -D_FORTIFY_SOURCE gcc flag. I fixed this bug and also tested xnec2c source code using cppcheck.

I decided it was about time I modify the xnec2c user interface so that it will, as far as possible, allow the user to save and re-open NEC2 files in the Editor window, without closing the Radiation Pattern and/or the Frequency Plots windows. This will significantly speed up work on editing NEC2 files and also make xnec2c usage less awkward. However, many bugs were introduced and fixed during this re-write of xnec2c, so users are advised to be watchful of possible bugs that got away and to report them for fixing.

Version 1.6

I received another bug report from Rik van Riel: The patch applied above did not help, as somehow the buggy code got duplicated below the bug fix, reproducing the same error in the calculation of front-to-back ratio! Hopefully fixed this time.

Version 1.5

Changed the handling of command line arguments so that the input file name may be specified without the use of the -i option. In this case xnec2c will take the last argument to be the input file name, but only if it has the .nec extension.Got rid of some variables that were set but not used, according to warnings given by gcc.Applied a patch supplied by Rik van Riel to allow the calculation of front to back ratios when the antenna is modeled over ground.

Version 1.4

Applied a patch supplied by Tom Beierlein, Tom, to fix crashing of xnec2c v1.3 on long input file names (> 80 characters).

Version 1.3

After an inquiry about using incident field excitation, which was not implemented in previous versions, I removed the restrictions in excitation to allow plotting the re-radiated pattern from a structure excited by incident field or elementary current source. However, the calculations are carried out only for one set of angle-of-incidence angles, e.g. there is no stepping of the θ and φ angles. This would require much more complex changes to xnec2c and I am not currently able to do this.

Version 1.2

Made the page size of spin buttons 0 to make setting of spin button values compatible with GTK 2.4

Version 1.1

I spoke too soon! A segfault bug has been reported that occurs when Maximum Admittance Matrix Asymmetry printing is requested in the EX card. Although this is not supported in xnec2c, it still produces a segmentation fault because the ipnt buffer is not allocated in the netwk() function in src/network.c

Version 1.0

After several months with no bug reports or feature requests, I am sticking my neck out and releasing xnec2c as v1.0. This version incorporates the last two feature request I received from users: The "Cancel" button on card editors has been replaces with a "Delete Card" button, which deletes the selected "card" (row in the NEC2 editor window). A "Net Gain" menu item has been added to the View menu of the Frequency Plots window to replace the second plotted quantity with the Net Gain of the antenna.

Version 1.0b5

Following on the above changes, I revised all similar situations in xnec2c's source and modified all sprintf commands to snprintf just in case, as I could not replicate the bug so could not test for other similar problems. I also fixed a bug in the "save" and "save as" handler, to avoid false attempts by xnec2c to save structure and radiation pattern/frequency plots pixmaps when a save of the NEC2 editor data failed for some reason.

Version 1.0b4

After a bug report from Juha Vierinen I changed some sprintf commands to snprintf to avoid buffer overruns.

Version 1.0b3

Added setlocale(LC_NUMERIC, "C"); to the main() function, so that in locales where commas are used in decimal numbers xnec2c can read data correctly (suggested by Joop Stakenborg). Some minor changes to the user interface were also made to allow the NEC2 editor window to fit in displays with shorter heights.

Version 1.0b

In this version xnec2c has been re-worked extensively to make it multi-threading and to streamline its operation to some extend. Many bugs created by these changes, and others that already existed have been fixed, and the user interface has also changed somewhat. The example input files have also been checked and some mistakes in them have been corrected. This is now the first v1.0 beta release for public testing.

Version 0.9b

Fixed a few bugs mainly in the NEC2 editor code and also changed the code of all individual Geometry and Command editors, so that edited data are set in the main NEC2 editor's tree views when another Geometry or Command card (tree view row) is selected for editing.

Implemented a work round around a serious bug which I could not trace, since it now seems to be outside my code: When the zint() or fbar() functions are called, they seem to corrupt xnec2c's memory allocation when they return. This corruption manifests itself as NAN values appearing in calculations so I modified these functions to return their computed value via a pointer in the argument list. I also changed the function declaration from complex long double to void as it seems this bug is related to functions of the former type declaration, returning a value.

Version 0.8b

Fixed a serious bug that caused segmentation faults and crashes in structures that have a wire in contact with a patch. The subph() routine divides this patch into 3 or 4 sub-patches so the total number of patches increases. There was no provision to reallocate and initialize buffers used in rendering patches in the structure display window. Other minor bugs were also fixed and the example NEC2 files were also checked and edited when needed.

Version 0.7b

Fixed a bug in the co-ordinate translation (Move) editor and edited the code in the GW card editor so that the wire diameter does not change when the New button is clicked.

Version 0.6b

Fixed a bug inherited from NEC2: If no geometry cards are present (only a GE card) then there is division by zero in conect(). NEC2 seems to accept the lack of geometry cards, this is now an error condition in xnec2c. Reduced the max value specification of the Capacitance spin button in the Loading card editor, from 1.0e+12 to 1.0e+9 (pF) since the higher value is beyond the range of the long type in 32 bit systems. Modified the behavior of some card Editors to make them more user friendly.

Version 0.5b

Fixed a bug that caused segmentation faults when only one wire segment was present in the structure. This is not a case that will normally exist but the seg fault had to be fixed.

Version 0.4b

Added a NEC2 input file editor that makes the edit/save/reload/execute cycle easier and quicker.

Version 0.3

Added the ability to stop the frequency loop by clicking on the frequency display spin buttons and to restart it by toggling the "Gain" or "E/H" buttons in the Radiation Pattern window.

Version 0.2

Incorporated some changes to the GUI (the Glade-generated design) since after upgrading to GTK+ 2.8.9, the geometry of the windows (the position and extend of buttons/entry widgets) changed a little. Also fixed some bugs in the GUI code to handle unusual sequences of user actions correctly.

Version 0.1

First version with ability to draw a color-coded wire frame Radiation Pattern (Near and Far Field), Graphs of various Frequency dependent data (Gain, VSWR, Impedance etc) and the Structure (Wires/Patches), including a color coded Current or Charge distribution.

Limitations

Since xnec2c is interactive, it cannot operate in the same way as NEC2 or nec2c. Specifically, commands that cause execution in NEC2/nec2c (XQ, RP etc), are only read in but not acted upon unless the user requests the display of relevant data. For example, if an RP command line is included in the input file, xnec2c reads the relevant data from that line but does not calculate/render the radiation pattern, until the user requests this by opening the Radiation Pattern window and clicking on the Gain button. In addition, the NX and WG/GF commands are not recognized since only one structure at a time can be input and evaluated, and the Numerical Green's function is not needed or implemented. Also, some options of certain commands (e.g.  the surface wave option I1 = 1 of the RP command) are not implemented and they must not be used since they will disrupt or even crash xnec2c.

There are advantages deriving from the interactive operation: it is possible, for example, to specify both the NE and NH commands in combination with a multiple-frequency FR card, although only the relevant data of the last command will be used.

No Output File

Printing of results to an output file has been removed starting from v1.0, since xnec2c works in a way that does not allow printing compatible with the original NEC2 output format. If printing to file is needed then it is better to use the original NEC2 program.

Bugs and Inadequacies

Xnec2c is based on nec2c, my translation to C of the original FORTRAN NEC2 code. Any bugs discovered in nec2c will affect xnec2c as well and they will have to be fixed. In addition, changing the flow logic of nec2c, from a non-interactive batch-processing command line tool to a GUI-based interactive application, was rather complex and introduced many bugs that were fixed, but it is always possible that a combination of some input file with an untested sequence of user actions may trigger a hidden bug. Such a case in fact did appear and it was traced to a bug in the original NEC2 code. This has been fixed by Gerald J. Burke (Lawrence Livermore National Laboratory, original FORTRAN NEC2 author) and the fix has been incorporated in nec2c and xnec2c. See the doc/NEC2-bug.txt file for details.

When xnec2c was made multi-threading, a lot of bugs appeared and were fixed but again there may be some that have not showed up. One condition that did appear a couple of times was xnec2c getting blocked in a select() call, waiting for a child to write to a pipe. This apparently happened because I was testing a very minimal input file and the child processes seemed to write to the pipes before the parent process dropped into select(). This was fixed in v4.3 by always sending full-length filenames to the job children. If this seems to happen, then please send a bug report with as much detail as possible to reproduce the problem.

A known inadequacy of xnec2c is the slowness of the animation of displayed drawings, e.g. the structure itself, the radiation pattern, near fields pattern etc. Specifically, dragging these drawings with the mouse to rotate or incline them seems very heavy on processor loading, and with most structures the movement is jerky. This is my first attempt at animated wire frame drawings and I lack experience with GTK3 in this field, so I probably went about this the wrong way. I am open to any suggestions that may solve this problem!