Date: Thu, 1 Nov 2007 08:27:40 +0000
From: John Pumford-Green GM4SLV <gm4slv@sighthound.demon.co.uk>
To: rsgb_lf_group@blacksheep.org
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Organization: The Gammy Bird
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Subject: Re: LF: RE: 500 kHz report
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On Thu, 01 Nov 2007 07:00:39 -0000
"Dave Sergeant" <dave@davesergeant.com> wrote:


> So my only problem is that most people can't hear me! But that is 
> down to the sand below the lawn and the corresponding astronomic 
> earth resistance. Only thing I can do there is to move!
>

Dave, LF,

The other option is more power of course! If your antenna is
inefficient due to low radiation resistance you need more current and
if it's poor because of high environmental losses you need more driving
power to get the current.

I tried a simple vertical 2 weeks ago in an attempt to eliminate the
top loading wires for the winter (got tired of them breaking free in
the gales). 

The antenna was 11m with no top-capacity loading and using a
professional FSM I found that with 80W TX power and 0.9A RF current
at the feed-point I got an ERP of only 85mW.

I relented and installed some top-loading. With 4 x 7m long capacity
loading wires running down the upper guys (not a long horizontal
stretch), and only 30W TX power, I achieved 140mW ERP.

The bare vertical was very inefficient due to low Rrad, as there was
plenty of current flowing, almost 1A. 

The top loading however gave 6dB gain in ERP! I also found the overall
R dropped considerably with the longer top-load wires (previously I'd
used 3.5m long loading wires), needing large change in matching
transformer taps to get back to 50 ohms. The overall losses must be
lower. Probably due in part to needing much less L to bring it to
resonance, and fewer losses in the loading coil.

If I couldn't improve the ground losses, which I find I can't either
(very wet peat on rock isn't great!) I'd put more power into the
antenna, which I do. I find that with a real "QRP" TX of ~5W I'd not be
heard by anyone except in the most perfect of conditions. At least you
have some locals to work I suppose. 

My nearest regular stations are Mal and Finbar - both over 600km away,
even Mark GM4ISM is 500km+ and he's a "GM"!

No-one will hear you if you can't radiate a signal!

I heard Chris G3XVL call/work you on Tuesday and strained my ears, and
eyes on the waterfall, but could see nothing more than the very faintest
hint of a signal between his overs. 


More power Igor!

-- 
John GM4SLV
IP90gg
Clousta, Shetland