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From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
References: <Zen-1HrBcp-0002Dj-3r@pythagoras.zen.co.uk>
Date: Thu, 24 May 2007 21:01:22 +0100
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Subject: LF: RE: 500kHz Distance
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Status: O

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