Reading the current series of postings on power and Antennas with insulators
catching fire makes it clear that we are talking about lethal power levels.
What are the safety issues and how should we protect ourselves, kids and
neighbours? Recent emails talk of direct rectification of mains and three
phase supplies! Its all getting very hairy.
Martin M5CIX
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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