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Old July 23rd 05, 01:23 AM
Roy Lewallen
 
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Richard Fry wrote:
"Richard Clark" wrote (among much else):

Try as I might in directions other than towards Puget Sound,
I will never launch any significant signals at angles lower
than this 15°.


_________________

Maybe you won't, but that doesn't mean it is impossible. A
well-designed, well-implemented vertical can do that. If it couldn't,
MW broadcast stations wouldn't have much of a groundwave.

RF


I'm sure Richard (Clark) is talking about sky wave. You're talking about
surface wave. Indeed, a field is "launched" at all angles. But the
portion at low angles (the surface wave) attenuates with distance, and
the attenuation increases with frequency. At HF and above it's good for
only a few miles, and all that's left beyond that is the sky wave, at
higher angles. It is possible to get very low angle sky wave, but it
requires vertically polarized waves and a very good conductor like salt
water for several wavelengths from the antenna in the direction of
propagation; favorably sloping terrain; or an extremely high
horizontally polarized antenna.

As I'm sure you know, AM broadcast antennas intentionally radiate very
little sky wave, and that's what amateurs need for communication beyond
a few miles. Some care must be used in comparing MW broadcasting
requirements and characteristics with amateur HF communications.

Roy Lewallen, W7EL


 
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