Similar arguments apply to early ships' aerials. The resonant frequencies
claimed, only seem to work out if one adds to the height and half the
horizontal of the aerial, ,the height of the metal deck/shack - which they
regarded as "Earth" - above sea level. The aerial resonance seems to have
determined the transmitted frequency ? by selecting from the broadband spark
emission. The whole hull was radiating.!!
G3GVB - Bryan
----- Original Message -----
From: "Peter Martinez" <[email protected]>
To: <[email protected]>
Sent: 23 June 2006 17:38
Subject: LF: Top-fed LF antenna idea
> 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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>
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