[Lf] Big vs. small antennas.

[Andre' Kesteloot]

Walter Blanchard wrote:

>  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!
>
> Happy New Year, Gluckliches Neues Jahr (sorry I don't know
> any Dutch, Swedish or Finnish!)
>
> Walter G3JKV.
>
>