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dB or so for HF stations close to the sea. I have not seen the original papers, but I have some familiarity with standard British Post Office installations of the classic era and can state that horizontally polarised antennas were almost always used above 3mHz. (Until recent times when things like discones came into play verticals were hardly ever used on HF). If the salt water is not a factor, then it must be assumed that other site factors such as ground slope are involved. I am not learned in wave propagation, but would presume from the above line of reasoning that the uncluttered Fresnel zone over water (as opposed to, say, a hilltop site looking over a hilly foreground) might be the determinant. Presumably the effective height of the seaside antenna would be better than one at equivalent elevation over a cluttered foreground? Some time ago RSGB 'Radio Communication' published an article comparing signal strengths in Australia from 40m G stations. While the data is scrappy, the top performers used either vertical arrays e.g Four Square, or two element inductively loaded shortened yagis. The best yagis were on elevated locations close to the sea. The best verticals were not necessarily close to the sea, but all seemed to be on flat lands. Perhaps someone better informed could comment. The effectiveness of vertically polarised antennas over saltwater is generally accepted, although my own limited experience suggests that the behaviour of hf vertical antennas in elevated locations is hard to predict. While generally mediocre, some seem to perform amazingly well. 73 Barry ZL1DD > end Barry (Baz) Kirkwood PhD ZL1DD ex ZL1BN, ZL4OK etc Signal Hill 66 Cory Road STOP AT SIGNAL HILL HOMESTAY B&B Palm Beach ON BEAUTIFUL WAIHEKE ISLAND Waiheke Island 1240 ONLY 35 MINUTES BY FERRY NEW ZEALAND FROM DOWNTOWN AUCKLAND Ph/Fax 64-9-372-5161 www.waiheke.co.nz/signal.htm -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact force12@qth.com |
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