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 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 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.
