Dear Fausto, LF Group,
Having the top loading wires as high up as possible is always best - greater
height means higher radiation resistance and usually lower loss resistance,
so that will boost your antenna efficiency. A rough guess would be that you
could get 15m effective antenna height or perhaps a little more; with Rrad
of 73 milliohms you would only need about 2.7A antenna current to reach the
1W ERP "legal limit". There do not seem to be many buildings and trees
around your QTH, so loss resistance should be fairly low - probably <50ohms.
So a few hundred watts TX power should be enough. A single downlead going
directly to the antenna tuner is a much better idea than a transmission
line, due to the fairly low antenna capacitance.
The voltage with 300pF antenna capacitance and Iant around 3A would be about
12kV. This requires care, but is quite workable. I used an antenna with
about 340pF capacitance (although only about 10m high) with antenna currents
up to 6A, which was quite reliable eventually. I did initially have several
problems with voltage brealdown. Initially, the loading coil was indoors,
with the antenna connected via a marine-type feed-through porcelain
insulator. The insulator was mounted on a plastic panel, and passed through
an 80mm diameter hole in some wooden cladding. This worked OK when the
weather was dry, but when the wood got damp in the rain, it caught fire!...
I also burned a number of plastic insulators supporting the antenna wire.
Eventually, the loading coil moved outside into its own shelter, and the
porcelain insulator was mounted through a hole bored in a glass cooking dish
about 250mm diameter, and this has proved a reliable insulator when exposed
to the weather. I stopped the plastic insulators melting by fitting "corona
rings" to the ends of the antenna wire to reduce the potential gradient.
Cheers, Jim Moritz
73 de M0BMU
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