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August 29th 04, 08:31 PM

Walter Maxwell

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On Sat, 28 Aug 2004 21:32:57 -0500, (Richard Harrison)
wrote:
snip
Unbalanced vertical antennas are convenient for groundwaves to extend
beyond the horizon for the mediumwave broadcaster. These antennas are
conveniently fed by coax of the concentric pipe or skeletal types. All
groundwaves are vertically polarized. These can travel very far at low
and medium frequencies. Attenuation of high frequency groundwaves is
severe. There is no propagation of horizontally polarized groundwaves at
all. The low-angle reflected wave is out of phase with the incident
wave.

Bottom line is that shortwave broadcasters transmit from horizontal
antennas and mediumwave broadcasters transmit from vertical antennas.
For shortwave, the target is reached via the ionosphere. For mediumwave,
the target is reached via the earth`s surface which is involved in
reaching beyond the line of sight.

Richard, when I was doing AM BC consulting more than 50 years ago I made the
observation from FCC ground-wave distance vs frequency propagation charts that
with the same ground conductivity and 1/4 wl antennas for both frequencies, the
1mv/m contour attained by 100 w at 540 kHz would require slightly less than 50
Kw at 1500 kHz,

Proves your point, doesn't it?

Walt, W2DU

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