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Hello gang,
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:
The "professional"
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. You can't do that with a piece of wire
waving in the wind; coils of unknown and variable characteristics, poor
earth systems, etc. 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. You want absolutely
stable characteristics whether it's wet, loaded with ice, covered in snow,
or blowing a Force 12 gale. 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. Automatic tuning can do a bit for you but not
everything.
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. Some high-powered Russian navigation transmitters have gone
off the air recently because they couldn't afford the electricity
bill. 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. 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.
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. 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.
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; 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. In other words, totally impracticable if you want
24hr availability. And yes, I have 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) it (sort of) worked, but it
never worked in England for more than a few minutes. Theory 10, practicality
0. 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).
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 us
"amateurs" are also "professionals" but find
"amateur" work liberating after the irritating constraints of a
lot of "professional" work.
My goodness, this bottle of
"Glenfiddich" has gone down rather quickly - and it's not even the
New Year yet!
I
wonder what you would be like on a good glass of Irish whiskey. It is a good
story but lacks the cartoon section that appeared in the old comic discussed
here recently.
Happy
New Year and try some Black Bush next time. You might publish a hard back
soon!!!!!!!!!!!!! You have potential and I hope a sense of HUMOUR which
sadly SOME seem to lack on here.
G3KEV
Happy
New Year, Gluckliches Neues Jahr (sorry I don't know any Dutch, Swedish or
Finnish!)
Walter G3JKV.
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