On the modeling of radio masts

[AN-SOF Antenna Simulator Group]

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 Configure 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.

Both models can be downloaded from: https://antennasimulator.com/.../BroadcastTowerRadials.zip