The Savièse main antenna simulation

In order to get a rough idea of the performance of such antenna, it has been simulated and the gain, the radiation pattern, the antenna impedance and the current distribution has been calculated. Precise data about the structure are not available and the dimensions have been estimated via observations on site. No antenna performance data is available and therefore the result of the simulation cannot be checked against the reality and may be considerably wrong, but the figures look plausible and are reported and commented below.

Antenna structure:

The main antenna of the Savièse transmitter is basically a top fed sloper mounted on a telecom tower. In other words the antenna is composed by a wire suspended at an existing telecom tower (and insulated from it) that goes down diagonally toward the ground. The wire is insulated from ground by a couple of insulators. The tower is grounded. The feed point is between the tower and the wire with an antenna tuner unit. Exact dimensions of this antenna are not available and, since it has been dismantled, it's not possible anymore to go on site and estimate any better. Transmission frequency is 1485 kHz, one wavelength is 202 m and a quarter wavelength is 50.5 m.

Antenna structure

Ground conductivity has a very big influence on the gain (the higher the conductivity the higher the gain), the elevation angle (the higher the conductivity the lower the angle) and the impedance. Unfortunately no information about ground conductivity is available. It's not known if this antenna buried radials or not, but in any case the terrain configuration suggest that a very good ground system is not present, so a value of 2 mS/m has been arbitrary chosen.

Simulation results:

First of all, the terrain configuration is not flat in this area and the real antenna wire ends below the base of the tower without being grounded. This, unfortunately, couldn't be modeled, so the slope of the wire was modified to fit on a perfectly flat ground. It's not known what influence this assumption may have. Then the simulator doesn't like the abrupt change in size and angle where the wire connects to the tower introducing further errors and, finally, the exact length of the wire is not known. For these reasons, the results of this simulation may be considerably wrong, but interesting enough to be worth mentioning.

Anyway, the simulation shows a gain of –0.5 dBd (including losses), an elevation angle of 28° and a half power beam-width of 51°. Because of the non-symmetrical structure of the antenna, it's slightly directive. The gain is maximum "behind" the tower and minimum in the direction of the wire. The difference in gain is 0.6 dB.

Vertical radiation pttern Horizontal radiation pttern

The calculated feed impedance is (40 – j142) Ω, meaning that for a power of 1 kW, the feed current is about 5 A and the feed voltage is 740 V. The impedance may vary according to the length of the wire, which is not exactly known, but the wire is about λ/4 so the impedance has to be a reasonable value in the order of 100 Ω. To have a comparison, 1 kW on a 50Ω resistive load requires a voltage of 223 V and a current of 4.47 A.

Antenna current distribution