[Fwd: LF: Polarisation]
Tue, 24 Feb 1998 08:34:55 -0500
Walter Blanchard wrote:
> Re G4GVC and G3XDV comments;
> The simple reason for professionals not using horizontally-polarised
> at LF/MF is that they produce a lot of skywave which is, to them, a blasted
> nuisance that causes fading and unpredictable service areas.
> They therefore take all possible steps to get rid of it by maximising
> groundwave/skywave ratio. It is a physical and demonstrable fact that the
> attenuation of horizontally polarised waves over the ground is far greater
> that of vertically-polarised, so the way is to use vertical polarisation.
> Hence nearly
> all the work has been done and textbook discussions centred on how to get the
> strongest possible vertically polarised groundwave. You will not find
> horizontally-polarised antennas being used for MW/LW BC purposes!
> The trouble is that this has been going on for so long with every budding
> radio engineer being taught it at an early stage that very few of them give
> thought to anything else. Hence the common belief that horizontally polarised
> waves are useless. It is really another manifestation of the belief amongst
> professionals in the early 1920's that shortwaves were useless because their
> groundwave range was so short and therefore they could be given away to
> quite safely!
> But this has nothing to do with which gives the greatest range if you are
> prepared to put up with fading and perhaps limit activities to only parts
> of the 24 hours. Amateurs have the terrific advantage they don't have to
> provide a 24-hr guaranteed service! A horizontal antenna is ideal for this
> situation - it shoves out large amounts of skywave even if somewhat
> unpredictably. We've been demonstrating this on 160m for many years.
> No respectable professional would dream of putting up a low half-wave
> for a 2 MHz system.
> Anyway, even if your "horizontal" antenna is only 30 or 40 feet high and
> it still has that 30 or 40 feet as a vertical component and the rest is
> loading. LF penetrates the earth quite well (cf. mine communications and
> in salt water you can receive LF signals down to 20-30 ft) so if you have
> no earth
> system your "virtual antenna" is probably buried some 50 ft or more, making
> vertical bit perhaps 100ft. If you then put in a copper mat on the surface
> you lose
> this and come back to the height of your wire, thus a weaker signal. So
> why do
> professionals put in their very expensive earth systems? Because the
> antenna" is totally unpredictable, varying with season, rainfall, water
> table height,
> earth conductivity, and so on.
> A little story to demonstrate this. In 1944, on an island in the Pacific,
> the US Navy
> put in a vertical antenna 125 ft high for the old Loran-A navigation
> system (the one
> that used to plague us on 160m). It was right on a very low-lying coast so
> to save
> time they thought they'd use the salt-water as an earth and sank a number
> of copper
> plates into the sea. It was useless - as the tide rose and fell the
> effective height
> of the antenna varied so much it couldn't be kept loaded properly. That
> the phasing of the system and its usefulness as a navaid. They had to
> re-site the
> antenna a little inland and put in a traditional copper mat..
> I've never known an amateur worry about that sort of thing!
> Walter G3JKV.