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From: "Peter Martinez" <peter.martinez@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Sat, 24 Jun 2006 06:31:24 -0000
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Subject: LF: Re: Re: Top-fed LF antenna idea
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>From G3PLX:

Dave/Brian/Jim/all..

Thanks for the feedback. I also received one from Bob VE7BS describing his 
pal's results from the top of a tower block with a whip on top-band.

Brian's point about ships antennas is one I also recall seeing mentioned in 
G3LNP's article about his top-fed tower. It seems that the early workers 
just calculated the effective height of the antenna measured from the radio 
cabin, not from the sea surface, and were surprised at how good the results 
were.  I think there has been a blind spot in conventional understanding of 
how a small antenna fed against 'ground' works when it's sitting ontop of 
something else, and it seemed to me that if we took this into account, a lot 
more interesting locations might suggest themselves for LF working.

We don't need to be able to actually see and touch the conductors involved 
in the process of carrying the current up and down the structure in order to 
know that we are using it as an antenna. If I sit on the top of my tower 
with some sort of whip or capacity hat out in clear space, and I can measure 
that I am poking a useful amount of RF current into it, then I don't need to 
go to great lengths to trace how this current is getting back down the tower 
to ground. In particular I don't need to run a wire down the side of the 
tower and worry about how to ground it at the bottom.  I can assume the 
current is flowing down the tower because there's simply nowhere else it can 
go. If a meter at the base of the whip registers 1 amp then that's 1 amp 
flowing down the whole height of the tower. I can then just measure the 
height of the tower above the surrounding terrain, derive the radiation 
resistance (using the full height, not half of it), and estimate the erp 
from that.  As I said in my RadCom letter, a small antenna fed 'against 
ground' at a height of H metres is a vertical antenna H metres tall.  This 
is clearly only true if the structures in question are small compared to a 
wavelength, but this is certainly going to be true for 2.2km waves.

What would be useful now is to see how easy or difficult it might be to push 
a decent amount of RF current into a whip from the top of something tall.

73
Peter