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From: "captbrian" <captbrian@ukonline.co.uk>
To: <rsgb_lf_group@blacksheep.org>
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Date: Sat, 24 Jun 2006 09:44:18 +0100
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Subject: LF: Re: Re: Re: Top-fed LF antenna idea
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Status: O

Why struggle with a short whip when on flat roof of many buildings is space
for a 70 foot  horizontal ?

How about avoiding falling off by laying the antenna  wire along a long
top-floor corridor?
 [let's not go on about floors again tho']

Book into a skyscraper hotel room on top floor, then ,when everyone asleep
.........heh.hehe,heh!

Bryan


----- Original Message -----
From: "Peter Martinez" <peter.martinez@btinternet.com>
To: <rsgb_lf_group@blacksheep.org>
Sent: 24 June 2006 07:31
Subject: LF: Re: Re: Top-fed LF antenna idea


> 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
>
>
>
>
>
> --
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>