The Monte Ceneri Cima di Dentro spare 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 spare antenna of the Monte Ceneri Cima transmitter is a short
vertical cage suspended between two metallic poles by a capacitive hat.
In spite of one could think it’s not a dipole. The base of the cage is
insulated from ground and connected to the feeder via a matching
network. 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. A value of
7.5 mS/m has been arbitrary chosen.
Simulation results:
The simulation shows a gain of 1.1 dBd (including losses), which is
not as good as the 1.6 dBd of the main antenna, but is still fair.
The elevation angle is 22° and a half power beam-width of 51°.
A very low radiation angle is important for long distance broadcast, but
such a small antenna can only achieve 22° (compared to the 14° of
the main antenna). The radiation pattern shows no side lobes at high
angles, meaning reduced fading in the reception, which is a very
important feature of a broadcast antenna, but still this antenna radiates
more energy toward the sky making it less performing against fading as
the main antenna.
The antenna is almost omni-directional in the horizontal plane; the two
poles only slightly modify the pattern by making the gain 0.5 dB
higher in the direction of the poles.
The current distribution on the mast is the typical one of short antennas
(shorter than λ/4). The maximum current is 260 A at the feed
point. The feed impedance is (4.5 – j41) Ω,
meaning that for a power of 300 kW, the feed current is about
260 A and the feed voltage is 10.7 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 capacitive hat?
The currents in the hat are in opposite directions and their radiation
cancels each other; almost no radiation comes from this part of the
antenna and this is proved by the fact that the antenna is almost
omni-directional in the horizontal plane.
So the question: how would this antenna perform without the capacitive
hat? In order to have a better comparison, the cage has been supposed to
be as high as the masts (in the real antenna the capacitive hat is tilted
and the cage is lower than the masts).
The simulation shows that the gain is lower (0.9 dBd), the elevation
angle is also lower (20°) and the half power beam-width a little
smaller (47°). But the big difference is in the feed impedance which
is now (2.2 – j370) Ω making a real challenge
to match this impedance to a regular 50 Ω transmission line.
The feed voltage would be around 138 kV and the current around
380 A, meaning an apparent power of 52 MVA for just 300 kW
active power. If such an antenna were constructed a great part of the
power would be wasted in the matching network.
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