From the far field point of view, the whole structure of an antenna and its surroundings is reduced to a single point at the origin (X,Y,Z) = (0,0,0). So the standard practice of superimposing the 3D radiation pattern to the antenna structure is just a means to facilitate the interpretation of the directional characteristics of an antenna.
For this reason, you can move the phase origin of the 3D radiation pattern to the desired point in order to get a better view of the antenna orientation versus its radiation pattern. Go to the Configure tab > Far Field > Origin and set the X0, Y0, Z0 coordinates of the radiation pattern center (see Section “4.3 Far-Fields” in AN-SOF user’s guide).
Wires can be imported into AN-SOF from another AN-SOF project, so wire structures of different projects can be merged in a single project.
When a project is saved, a file having extension .wre will also be saved. This file contains the geometrical description of the wires. To import wires to a project, go to File menu > Import Wires > AN-SOF Format and just find and select the .wre file that you want to import.
For instance, this feature allows us to analyze the electromagnetic response of an antenna and its supporting structure separately, and then to combine them in a new project to analyze the response of the whole structure.
Radiating towers or radio masts can be modeled in AN-SOF with a high degree of detail, as shown in this figure. Since we already know the omnidirectional shape of the radiation pattern, what interests us is to calculate electric field values at ground level for a given input power. Go to Setup tab > Near Field panel and set the desired coordinate system (Cartesian or Cylindrical: Z = 0, Spherical: theta = 90°). Go to the Excitation panel to set the input power (it is customary to set 1,000 W). To increase the conductivity of the soil and therefore the radiation efficiency, we can use a radial wire ground screen.
Regarding the feeding point, we can put a source at the position of the base insulator if the feedline will be connected there in the real life antenna. In this way, we will obtain the input impedance of the antenna, which we can then post-process (e.g. tuning house + transmission line coming from the transmitter).
To speed up the simulation, we could use a simplified model, which consists of a single vertical wire with a triangular cross section. The radiation pattern will be the same as before, but we must be careful at the feeding point.
The base of the tower in the detailed model forms a short transmission line to the source position at the tower center. In the simplified model, we can offset the source from the center a distance equal to the half width of the tower to simulate the short transmission line effect.
We see that most of the time we are interested in calculating only the E-field in antenna projects when we are talking about the near field. For this reason, we have added an option to enable or disable the automatic calculation of the H-field when we click on the “Run ALL” (F10) button. Go to Tools > Preferences > Options tab.
If we are only interested in the near field and we don’t want to waste time calculating the far field, we can click on the “Run Currents and Near-Field” (F12) button.
In the “Simulate” menu we also have the options: “Run Far-Field”, “Run Near E-Field” and “Run Near H-Field” to calculate each field separately.
There is always a trade-off between speed and accuracy. However, we often need to prioritize speed in the first simulations of an antenna model. Here are some tips to speed up the calculations.
- Start with the minimum number of segments (10 segments per wavelength). AN-SOF sets the minimum number if you write “0” (zero) as the number of segments on any wire. In a wire grid, use one segment per cell side if the electrical size of each cell is < 10% of the wavelength.
- Set the Quadrature Tolerance between 5% and 10% in Setup tab > Settings panel. This parameter only has an appreciable effect when there are parallel wires very close to each other (about two wire radii apart).
- Go to Setup tab > Settings and set Interaction Distance = 0. It also only affects parallel wires that are very close to each other.
- To calculate the gain, directivity and efficiency usually it is enough to set a spatial resolution of 5 or 10 deg in the far-field. Setup tab > Far-Field panel > Theta Step = 5 and Phi Step = 10 deg.
- Last but not least, just calculate what you really need. The “Run ALL” option will calculate everything, currents, far and near fields. If you only need the currents and the far-field, click on “Run Currents and Far-Field (F11)”. If you are only interested in the input impedance, go to Run > Run Currents in the main menu.
The example in this figure shows that we can run a simulation more than 3 times faster, especially by setting the Interaction Distance to zero. You will find the “car.emm” project in the Examples folder which is installed together with AN-SOF.