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| To: | [email protected] |
|---|---|
| Subject: | LF: Big/small antennas. |
| From: | "Walter Blanchard" <[email protected]> |
| Date: | Wed, 27 Dec 2000 14:08:35 +0000 |
| Reply-to: | [email protected] |
| Sender: | <[email protected]> |
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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! Happy New Year, Gluckliches Neues Jahr (sorry I don't know any Dutch, Swedish or Finnish!) Walter G3JKV. |
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