Message-ID: <5.0.2.1.2.20001227104754.00a1bec0@mail.pncl.co.uk>
Date: Wed, 27 Dec 2000 14:08:35 +0000
To: rsgb_lf_group@blacksheep.org
From: "Walter Blanchard" <blanch@pncl.co.uk>
Subject: LF: Big/small antennas.
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Hello gang,<br>
<br>
Since our professional controversialist ('3KEV) has raised the big/small
antenna discussion again, and I'm fed up playing Scrabble, here's a few
points he might like to ponder on while he's drinking in the New
Year:<br>
<br>
The &quot;professional&quot; antenna/system engineer's job is to provide
a guaranteed signal at all times 24hrs/day, 365 days a year. If it is a
navigational system, as we had in Decca, not just guaranteed field
strength but guaranteed phase stability as well, to better than 10
degs.&nbsp; You can't do that with a piece of wire waving in the wind;
coils of unknown and variable characteristics, poor earth systems,
etc.&nbsp; And you want to minimise skywave as much as possible; it's
just a nuisance. So you go for a vertical, the bigger the better, not
particularly because it radiates better but because the bigger it is the
more predictable and stable it is.&nbsp; You want absolutely stable
characteristics whether it's wet, loaded with ice, covered in snow, or
blowing a Force 12 gale.&nbsp; And you don't want to pay for a team of
skilled engineers to be permanently on site 24 hrs/day to re-tune it
every time something changes.&nbsp; Automatic tuning can do a bit for you
but not everything. <br>
Then there's cost. A big antenna costs a lot in initial purchase and
annual maintenance, sure, but not as much as installing a 2 megawatt
transmitter and paying for its maintenance and the electricity it
consumes.&nbsp; Some high-powered Russian navigation transmitters have
gone off the air recently because <i>they couldn't afford the electricity
bill.&nbsp; </i>You have to look at where your income is coming from and
whether it will support the running costs of the high-powered transmitter
you're going to have to put in because you're only going to have a little
aerial.&nbsp; Years ago I used to have to do this sort of calculation and
believe you me the big antenna is cheaper and a good deal less trouble in
the long run. <br>
<br>
This is apart from the sheer engineering consideration of how much field
strength you need and where. The BBC consider that at LF they need a
minimum of 1 millivolt/metre to get a signal into a little tranny in a
block of flats in a city.&nbsp; This is about S9+60 db by amateur
standards (please, no arguments about what amateur S-points mean!) and
results in things like the 400 kW e.r.p Droitwich transmitter to serve
only a 200-mile radius but which can be heard in America quite
often.&nbsp; Note, incidentally, that as pointed out by others, range
depends ONLY on e.r.p. in the direction you want, it doesn't matter how
you produce it.  And yes, if it was possible to get 1W radiated out of a
10 foot vertical antenna at 136 kHz it would work just as well as 1W out
of a 600 ft vertical. At 136 kHz all antennas are tiny and radiate
omni-directionally in the vertical plane. The differences would be you
would need something like a 25 kW transmitter instead of a 2.5W tiddler;
as soon as it got foggy everything would arc over;&nbsp; it would be
impossible to keep loaded efficiently even if you sat with one hand on a
tuning control, you would have to spend a lot of money on very good
insulators, (and go out and polish them every few hours), etc, etc.&nbsp;
In other words, totally impracticable if you want 24hr
availability.&nbsp; And yes, I <i>have</i> tried it, about 30 years ago
we tried to radiate a 100 kHz signal off a 10 ft stick (coil, actually) ;
in perfectly dry conditions (Arizona desert)&nbsp; it (sort of) worked,
but it never worked in England for more than a few minutes. Theory 10,
practicality 0.&nbsp; As a matter of interest we eventually made the
system work reliably around the North Sea by using 180 footers (it was a
variety of Loran-C in case anyone's interested - closed down some years
ago in favour of DGPS). <br>
<br>
As amateurs we might well decide it's worth living with a lot of this
aggro just to get those few minutes across the Atlantic, but
professionally you can forget it. So please stop making silly comparisons
between amateur and professional practice. Quite a lot of&nbsp; us
&quot;amateurs&quot; are also &quot;professionals&quot; but find
&quot;amateur&quot; work liberating after the irritating constraints of a
lot of &quot;professional&quot; work. <br>
<br>
My goodness, this bottle of &quot;Glenfiddich&quot; has gone down rather
quickly - and it's not even the New Year yet!<br>
<br>
Happy New Year, Gluckliches Neues Jahr (sorry I don't know any Dutch,
Swedish or Finnish!)<br>
<br>
Walter G3JKV. <br>
<br>
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