Message-ID: <21206.200012061423@gemini>
From: "James Moritz" <j.r.moritz@herts.ac.uk>
Organization: University of Hertfordshire
To: rsgb_lf_group@blacksheep.org
Date: Wed, 6 Dec 2000 14:26:44 +0000
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Subject: LF: High Power, Antennas and Loading Coils
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Reply-To: rsgb_lf_group@blacksheep.org
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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