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From: "james moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Date: Fri, 23 Jun 2006 18:19:57 +0100
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Subject: LF: RE: Top-fed LF antenna idea
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Dear Peter, LF Group,

As I recall, G0MRF's first trans-atlantic transmission was done from the =
top
of a tower block with a down-sloping wire as "top loading" - Perhaps =
David
would care to fill in the details...

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Peter Martinez
Sent: 23 June 2006 17:38
To: rsgb_lf_group@blacksheep.org
Subject: LF: Top-fed LF antenna idea

>>From G3PLX:

Some of you may have seen my letter in the current RadCom, which =
suggests=20
that reports of unusually good performance of the electrically-small =
'EH'=20
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=20
someone is able to operate 'portable' from the top of a suitable =
structure,=20
like a telecom tower or the Eifel or Blackpool towers. What would be =
needed=20
would be some sort of capacity-hat, such as a fishing rod (or two) =
sticking=20
out into clear space from the top of the tower. It need not be vertical, =
so=20
long as most of it was clear of the structure. This would then be =
resonated=20
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=20
should be AC power up there from which to run the equipment, and the =
safety=20
ground wire of this should be sufficient, or there may be an acessible=20
lightning conductor.

The capacity hat would inevitably be quite small and therefore the =
inductor=20
losses will be high, but against that there will be an enormous benefit=20
because a lot of the 'resistance' in the ground connection will be good=20
quality radiation resistance. We should be able to calculate what this =
is=20
from the height of the structure, and remember that we can assume that =
the=20
current up the tower will be fairly constant and not dropping linearly =
to=20
zero at the top like a base-fed tower would be, so the effective height =
is=20
the actual height, not half of it. The e.m.r.p. can then be calculated =
from=20
the product of this radiation resistance and the square of the current=20
flowing into the capacity hat.

I am sure the principle is sound. Is it worth a try?

73
Peter