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.
The main antenna of the Sottens transmitter is a vertical half wave dipole, fed at its center and supported by a 188 m high self supporting tower. The base of the tower is grounded. The dipole is composed by six wires suspended around the tower and connected to it at its top. The matching network is at the base of the tower and is connected to the feed point of the dipole (which is at 96 m above ground) via a 120 Ω cage coaxial cable inside the tower. The lower end of the dipole is connected to ground via a variable inductor that allows adjusting the phase in the lower arm and therefore modifying the radiation pattern. The antenna is equipped with a ground plane composed by 120 buried wires 190 m long. Transmission frequency is 765 kHz, one wavelength is 392 m and a quarter wavelength is 98 m.
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. Since this antenna has buried radials to increase ground conductivity, a value of 10 mS/m has been arbitrary chosen.
No information is available about the variable coil connecting the lower dipole arm to ground, so it has been experimentally adjusted until a good compromise between gain, elevation angle and side lobes has been achieved. It seems that with an inductor of 110 μH the results are pretty good.
The simulation shows a gain of 1.5 dBd (including losses), an elevation angle of 12° and a half power beam-width of 25°. A very low radiation angle is important for long distance broadcast. The radiation pattern also shows no side lobes at high angles, meaning reduced fading in the reception, which is a very important feature of a broadcast antenna.
The current distribution on the dipole is different from a "regular" λ/2 dipole because of the ground connection via the variable inductor of the lower end and because of the connection to the tower of the upper end. When the coil is properly adjusted, there is a current minimum of 1.5 A at about 12 m above ground. The current is maximum (82 A) right below the feed point at 96 m above ground. This particular current distribution is usual in broadcast tower since it makes a very good radiation pattern.
The feed impedance is (90 + j19) Ω, meaning that for a power of 600 kW, the feed current is about 82 A and the feed voltage is 7.5 kV. To have a comparison, 600 kW on a 50Ω resistive load require a voltage of 5.5 kV and a current of 110 A. This impedance is intended at the base of the tower, where the matching network is located, and takes therefore into account the impedance transformation of the 96 m long 120 Ω coaxial cable.
The coil at the lower end of the dipole can make the lower arm electrically longer and produces a current minimum near the base. The position of this minimum can be adjusted by varying the inductance of this coil. This has a significant effect on the radiation pattern and on the feed impedance.
The following radiation pattern is what happens if the coil is reduced from 110 μH to 60 μH. The gain remains at 1.5 dBd, the elevation angle drops to 10° and the half power beam-width shrinks to 18°. Since the current minimum is now further above ground, side lobes this high angles are now present. These side lobes are responsible for interference (fading) in the reception and are therefore always minimized. The feed impedance is now (2 + j24) Ω.
If, on the other hand, the inductance is increased from 110 μH to 200 μH, the radiation pattern can be observed in the following plot. The gain drops to 1.1 dBd, the elevation angle rises to 13° and the half power beam-width increases to 29°. Since the current minimum is very close to the ground, no extra side lobes are visible. The feed impedance is now (44 – j31) Ω.
