Dear LF Group,
Trying to run high power into a small antenna puts a lot of strain on
the loading coil and the antenna itself. For instance, my antenna
has loss resistance of about 30ohms. My original, small, loading
coil had a loss resistance of about 20ohms. My new loading coil,
made from salvaged Decca loading coil litz, has a loss resistance
of about 5ohms. The old loading coil sounds like it would lose a lot
of signal, but in fact the difference only works out to around 1.5dB -
probably why many people have concluded that huge loading coils
don't make a worthwhile difference. But the difference in power
dissipation is more significant - with 1.2kW TX output, the big coil
dissipates about 180W, as against 500W for the small coil, which
would probably lead to it's rapid demise.
Another factor to be considered is the antenna voltage. With about
6A antenna current, and 300pF capacitance, the antenna voltage
should be about 23kV. When I first started operating at this level of
power, I had problems with arcs forming on the antenna wire,
insulators catching fire, etc. Changing to a thicker antenna wire,
and fitting "corona rings" to the insulators, with the object of
reducing the potential gradient, seems to have solved the problem
for the time being.
Operating with a higher power level still has it's attractions - at the
moment, to get something like 1W ERP, I have to resort to using a
hydraulic cherry picker to support my antenna at 16m high - but I
can only do this during the night, in order to preserve relations with
the neighbors. With the antenna at it's daytime height, I reckon 2-
4kW would be required. But the antenna voltage would then be
between 30 and 50kV. 73kHz doesn't bear thinking about!
So there is a dilemma - building a huge amplifier as Andy describes
would require a bigger antenna with a lower reactance to keep the
voltage to a sensible level, but having a bigger antenna would
means less power is required (lower loss and higher radiation
resistance). Trying to increase the size of my antenna is
problematic for the usual reasons, so I suppose I am near the limit
of what ERP is possible from my QTH, for the time being.
Cheers, Jim Moritz
73 de M0BMU
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