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Quick Start Guide

Antenna Modeling Software

An antenna model is a representation of a real-world antenna in a computer program. This kind of model should not be confused with a scale model that sometimes is built to measure the radiation characteristics of an identical antenna with a larger physical size. Due to the complexity of the math involved in a model, a computer software is often programmed to predict and analyze antenna performance.

Computer simulation in industry is used to overcome challenges and drive innovation in the product creation and development processes. A computer model has the advantage that it can be modified, redesigned, broken, destroyed, and built again many times without wasting materials. Therefore, a considerable reduction in the cost of building successive physical models can be obtained during the design process with the help of a simulation software.

The geometry of the structure can be easily drawn in AN-SOF using the mouse, menus, and friendly dialog windows. Wires are drawn in 3D space where tools are available to zoom, move, and rotate the structure.

To plot the results from a simulation, a suite of applications has been integrated that allow us to display graphs: AN-XY Chart >, AN-Smith >, AN-Polar >, and AN-3D Pattern >. These tools can also be executed independently for a subsequent processing of graphics.

AN-SOF is the easiest to use software tool for the simulation of wire antennas and, at the same time, it is the most accurate one. The key advantages can be summarized as follows:

Note

AN-SOF has…

  • Fast and easy input and output graphical interfaces.
  • An extended frequency range.

AN-SOF can be used to…

  • design better antennas
  • predict antenna performance
  • tune for performance
  • account for environment effects
  • optimize a design using scripts
  • get insight into the behavior of an antenna
  • try many times before building the real model
  • learn more about antennas and share our findings with colleagues.
  • enjoy this exciting field!

AN-SOF allows us…

  • to describe the geometry of the antenna
  • to choose construction materials
  • to describe the environment and ground conditions
  • to describe the antenna height above ground
  • to analyze the radiation pattern and front-to-back ratio
  • to plot directivity and gain
  • to analyze impedance and SWR (Standing Wave Ratio)
  • to predict bandwidth

and to get many more parameters and plots.

Fundamentals of simulation

AN-SOF computes the electric currents flowing on metallic structures, including antennas in transmitting and receiving modes as well as scatterers. A scatterer is any object that can reflect and/or diffract radiofrequency waves. For example, the scattering of waves could be analyzed on the surface of an aircraft to investigate the best placement of an antenna, on a parabolic reflector to analyze gain as a function of the reflector shape, on the chassis of a car to predict interference effects, etc.

One of the most validated methods for antenna simulation is the so-called Method of Moments (MoM). An improved and advanced form of this method has been implemented in AN-SOF to overcome various well-known difficulties of the traditional MoM.

According to the MoM, any metallic structure can be modeled using conductive wires, as Fig. 1 shows. These wires must be divided into small pieces called segments. A wire segment has the shape of a cylindrical tube whose length should be short compared to the wavelength to get accurate results, Fig. 2. However, this is not a matter to worry about in a first simulation since automatic segmentation of wires is set by default in AN-SOF. Electric currents can be forced to flow on the structure by placing a voltage generator at some position that works at a given frequency. Current generators can also be used as the excitation, as well as a plane wave impinging on the structure that comes from a far or distant source.

Fig. 1: Computer models of a car, a parabolic reflector, a plane, and a ship using wire grids.
Fig. 2: A straight wire divided into short segments.

Once the structure geometry, materials and sources have been defined, the calculation can be run to obtain the currents flowing on the wire segments. In general, the electric currents will have varying intensities along and across the structure, so they are collectively referred to as a current distribution. Figure 3 shows an example of the current distribution on a log-periodic antenna.

The electromagnetic field radiated by the current distribution can be calculated in a second step of the simulation process. However, the current distribution itself gives a lot of information about the behavior of the structure, especially if a frequency sweep has been performed. In the case of antennas, the feed point impedance can be obtained as a function of frequency to analyze the bandwidth. The VSWR (Voltage Standing Wave Ratio) can be plotted in a Smith chart for a better interpretation of the results, Fig. 4.

Fig. 3: Current distribution on a log-periodic antenna. The color map on the structure indicates the amplitudes of the electric currents.
Fig. 4: Impedance plotted as a function of frequency in a Smith Chart, where the VSWR can be obtained by clicking on the curve.

The electric and magnetic fields can be obtained in the proximity of the structure, in the so-called near-field zone, and plotted as a color map whose intensities sometimes resemble the temperature maps in weather forecasts, Fig. 5.

Fig. 5: Near electric field in the proximity of a Horn antenna.

Far away from the structure, at several wavelengths, the magnetic field becomes proportional to the electric field, so only the electric field intensities are often used to analyze the results. This is the so-called far-field zone, where the radiated field is usually plotted as a function of direction in a polar diagram, Fig. 6. A more complete representation is obtained plotting a 3D pattern, where radiation lobes can be superimposed to the structure geometry for a better visualization of its directional properties, Fig. 7.

Fig. 6: Far-field pattern represented in a polar diagram. Beamwidth and front-to-back ratio are shown.
Fig. 7: Far-field pattern represented in a 3D plot and superimposed to the antenna geometry.

Performing the First Simulation

Several example files are included in the AN-SOF installation directory within a folder named “Examples”. Opening a file with extension “.emm” will show the wire structure on the screen. The calculation can be run by clicking on the Run ALL button > on the toolbar. The main results can be plotted by clicking on the Plot Current Distribution > button, the Far-Field 3D Plot > button, and the Far-Field Polar 1 Slice > button.

As a first experience using AN-SOF, a simulation of a standard half-wave dipole could be performed since this is one of the simplest antennas that can be modeled. A dipole is just a straight wire fed at its center. When the wire cross-section is circular, the dipole is called a cylindrical antenna. Since the material the wire is made of is usually a very good conductor, the wire can be considered a perfect conductor, that is, a material that has zero resistivity. Therefore, a cylindrical antenna with zero resistivity will be modeled in this example.

The first step is to set the operating frequency. Go to the Setup > tabsheet in the AN-SOF main window. In the Frequency > panel, three options can be chosen. Select Single and then write the operating frequency for the antenna, Fig. 8. In this case, the frequency is given in megahertz (MHz) and lengths are measured in meters (m). Go to Tools > Preferences > to change the unit system for frequencies and lengths if desired. Note that for a frequency of 300 MHz, the wavelength practically equals 1 meter (0.999308 m).

Once the operating frequency has been set, the antenna geometry can be drawn in the Workspace > tabsheet. The workspace is the place on the screen where the wire structure is drawn; it represents the 3D space where the structure can be zoomed, rotated, and moved.

Fig. 8: Single Frequency option in the Setup tab where a frequency of 300 MHz is set.

A straight wire is called a Line in AN-SOF. Go to Draw > Line > in the main menu. The Draw dialog box will be shown. In the Line tab, the coordinates of two distinct points can be set. In this example, the line will be along the z-axis and will be 0.5 meters long, which corresponds to half a wavelength at 300 MHz. Figure 9 shows that the starting point of the line is chosen at (X1,Y1,Z1) = (0,0,-0.25) m while the ending point is at (X2,Y2,Z2) = (0,0,0.25) m.

Fig. 9: Line tab in the Draw dialog box for drawing a straight line.

Then, go to the Attributes > tab, Fig. 10. The line must be divided into segments, which must be short compared to the wavelength. Basically, if the segment length is equal or less than a tenth of a wavelength, it is considered as a short segment. AN-SOF automatically suggests a minimum number of segments to achieve reliable results. To get more resolution, the number of segments can be increased. In this case, the line will be divided into 17 segments. The wire cross-section will be circular with 5 millimeters in radius. In the Materials > tab the wire resistivity will be set to zero, Fig. 11.

The next step is to feed the dipole. Right click on the wire and select the Source/Load command from the displayed pop-up menu >. A toolbar > with a slider will be displayed at the bottom of the screen. Move the slider to the segment at the wire center. Then, press the Add Source button. Add a voltage source 1 Volt in amplitude and zero phase, Fig. 12.

Fig. 10: Attributes tab in the Draw dialog box where the number of segments and wire radius can be set.
Fig. 11: Materials tab in the Draw dialog box for setting the wire resistivity.
Fig. 12: Add Source dialog box shown after pressing the Add Source button in the Source/Load toolbar at the bottom of the screen.

Go to Run > Run Currents > in the main menu to run the calculation. Once the calculations are done, go to Run > Run Far-Field > in the main menu. In this way, the current distribution on the dipole antenna and the radiated field will be calculated.

AN-SOF has integrated graphical tools > for the visualization of the results. Right click on the wire and select Plot Currents > in the displayed pop-up menu. A plot of the current distribution in amplitude along the dipole antenna will be shown, Fig. 13. Since a half-wave dipole has been drawn, the resulting current distribution is a semi-cycle approaching a sine function.

Several parameters from the point of view of the voltage source connected to the antenna terminals can be obtained. Right click on the wire and select List Currents in the pop-up menu. Move the slider to the position of the voltage source and click on the Input List > button. The input impedance of the dipole antenna will be shown and many other parameters, Fig. 14.

The input impedance can also be obtained by just clicking on the List Input Impedances (Zin) button in the main toolbar.

Fig. 13: Current distribution along a half-wave dipole.
Fig. 14: Input List dialog box where the input impedance can be seen.

The radiation pattern can be represented in a 3D plot. Go to Results > Plot Far-Field Pattern > 3D Plot > in the main menu. The normalized radiation pattern will be displayed in the AN-3D Pattern application. A color bar-scale indicates the field intensities over the radiation lobes. The directivity, gain and electric field patterns can also be plotted by going to the Plot menu in AN-3D Pattern. The half-wave dipole is an omnidirectional antenna in the plane perpendicular to the dipole axis (xy-plane), Fig. 15.

Fig. 15: Radiation pattern of a half-wave dipole.

Summary

In summary, simulating a wire structure is a three-step procedure:

1. Setup > Set frequencies, environment, and desired results.

2. Draw > Draw geometry, specify materials and sources.

3. Run > Run the calculation and visualize the results.

A convenient unit system for the frequencies and lengths can be chosen at the beginning of the simulation and can then be changed at any time by going to Tools > Preferences >. For example, the wire lengths are often measured either in meters (m) or feet (ft) at frequencies below 100 MHz, while either millimeters (mm) or inches (in) are preferred at higher frequencies.

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