From G3PLX:
Some of you may have seen my letter in the current RadCom, which suggests
that reports of unusually good performance of the electrically-small 'EH'
antennas may be the result of them being mounted on top of a tall structure,
and are effectively top-feeding the whole thing.
It occurs to me that this might be a useful experiment to try on 136kHz if
someone is able to operate 'portable' from the top of a suitable structure,
like a telecom tower or the Eifel or Blackpool towers. What would be needed
would be some sort of capacity-hat, such as a fishing rod (or two) sticking
out into clear space from the top of the tower. It need not be vertical, so
long as most of it was clear of the structure. This would then be resonated
at the operating frequency and the whole thing fed against some metallic
part of the structure. Assuming the tower is one with public access, there
should be AC power up there from which to run the equipment, and the safety
ground wire of this should be sufficient, or there may be an acessible
lightning conductor.
The capacity hat would inevitably be quite small and therefore the inductor
losses will be high, but against that there will be an enormous benefit
because a lot of the 'resistance' in the ground connection will be good
quality radiation resistance. We should be able to calculate what this is
from the height of the structure, and remember that we can assume that the
current up the tower will be fairly constant and not dropping linearly to
zero at the top like a base-fed tower would be, so the effective height is
the actual height, not half of it. The e.m.r.p. can then be calculated from
the product of this radiation resistance and the square of the current
flowing into the capacity hat.
I am sure the principle is sound. Is it worth a try?
73
Peter
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