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To: <rsgb_lf_group@blacksheep.org>
From: John W Gould <jwgould@iee.org.uk>
Date: Thu, 24 May 2007 20:25:05 GMT
Message-ID: <Zen-1HrJrt-0003cc-8z@heisenberg.zen.co.uk>
Subject: Re: LF: RE: 500kHz Distance
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Thanks Jim, I tend to agree with you, but I'm not quite sure whether you are supporting the 20W power to the aerial limit, with the additional ERP ceiling that Rik was proposing.  

Apologies for not understanding, but I'm feeling jaded having spent the day digging trenches and about 2 hours networking my laptop to my daughter's PC to get Internet access!

73 John, G3WKL

James Moritz <james.moritz@btopenworld.com> wrote :

> Dear John, Rik, LF Group,
> 
> I would agree with Rik's comments. Experimentation on 136k (and 73k) over
> the last several years has given us a pretty good idea of how to optimise
> the efficiency of small LF antennas, but it has also shown that, once "best
> practice" has been employed, one quickly reaches a point of diminishing
> returns where further improvements have negligible effect on the radiated
> signal level. Beyond this, significant improvement in antenna performance
> can only really be achieved by increasing the physical dimensions of the
> antenna, which is usually not feasible for non-technical reasons. The same
> is certainly true for 500kHz, although the antennas are effectively a bit
> better to begin with. So however much one may strive  to improve antenna
> efficiency, and whatever incentives there are, beyond a certain point it
> just isn't possible without discovery of some revolutionary and unknown new
> antenna design principle, which one cannot rely on ever happening (I
> hesitate to mention "Poynting vector synthesis"...). The 1W ERP limit on
> 136k has given us a fair amount of freedom and flexibility and has led to
> some interesting technical challenges and solutions (e.g. how to
> economically generate a lot of TX power and feed it into a tiny antenna
> without something going up in smoke :-) ) - I don't think the last several
> years on LF would have been nearly so rewarding if TX power had been limited
> to 10 or 20 watts as is currently being suggested for 500kHz.
> 
> Cheers, Jim Moritz
> 73 de M0BMU