The MF antenna Vatican (1530 kHz) consists of four towers (height appr. 0.5 lambda), each surrounded by a cage of six ropes. The towers are arranged as a
square with a spacing of 0.36 lambda. All towers are center fed.
The feeding voltages of the towers ( amplitude and phase ) are varied in order to achieve five different patterns.
The antenna is designed for little vertical radiation in the range of 0 to 30 degrees elevation angle of the vertical pattern. Therefore a coverage
of fading free groundwave propagation up to 100 km
from the Vatican site of Santa di Maria di Galeria
( Northwest of Rome ) is given.
Advantages of center fed antennas
When using base fed antennas, it is impossible to obtain a vertical pattern with a deep null and little vertical radiation in the range of 0 to 30 degrees elevation angle. In
this case the null of the pattern is filled up due to the fact that in addition to the nearly sine-shaped current distribution having a current null near the base of the tower, a considerable square term of current exists, which is necessary for the transport of power and which decreases slowly from the bottom to the top.
It should be mentioned that the vertical radiation in the range between 0 and 30 degrees (elevation angle) increases dependently on the thickness of the radiator.
In case of the center fed tower as existing in Vatican a vertical pattern with a very deep null and a small side lobe is achieved. The vertical pattern especially the position of the null depends on the current distribution on the radiator, in particular on the height of the minimum of current above ground.
Using base fed radiators this height can only be varied by altering the height of the antenna, which in practice usually is not applicable because of the great effort necessary.
The center fed radiator allows to modify the height of the minimum of current above ground by tuning the effective base reactance obtained by the lower rope cage. This can be done rather easily by adjusting the lower short circuit between tower and rope cage.