The Monte Ceneri Cima di Dentro 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
observation 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 Monte Ceneri Cima transmitter is a thin tower
220 m high. Its base is insulated from ground and connected to the
feeder via a matching network. The mast is hold in place by three groups
of 4 guy cables. All guys are grounded on the lower end and insulated from
the tower with an insulator that is placed almost against the tower
(longest guys) or some way down (shortest guys). Transmission frequency is
558 kHz, one wavelength is 538 m and a quarter wavelength is
134.4 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.
Simulation results:
The simulation shows a gain of 1.6 dBd (including losses), an elevation
angle of 14° and a half power beam-width of 34°. 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 antenna is almost omni-directional in the horizontal plane; the guy
wires only slightly modify the pattern by making the gain 0.5 dB higher
in the direction of the guys.
The current distribution on the mast is similar to a λ/2 dipole. The
maximum current is 50 A at about 90 m above ground. The feed
impedance is (307.5 + j133.5) Ω, meaning that for a
power of 300 kW, the feed current is about 31 A and the feed
voltage is 10.4 kV. To have a comparison, 300 kW on a 50Ω
resistive load require a voltage of 3.9 kV and a current of
77.5 A.
What if the antenna had no guys?
The simulation shows that a significant current flow in the guy wires, its
maximum is in the second shortest guy wires at the ground connection where
the current is about 18 A (per wire), but its influence on the antenna
performance is small. The antenna has also been simulated without guy wires:
the gain is slightly higher (1.9 dBd), the elevation angle stays the
same (14°) and the half power beam-width a little smaller (32°). The
feed impedance, on the other hand, is considerably different
(544.2 + j269.2) Ω changing feed voltage and current to
14.3 kV and 23.5 A for the same power. The current distribution
on the mast is similar, but this time the maximum current is 58 A at
about 96 m above ground.
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