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Mountains don't simply block HF

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February 13th 11, 08:59 PM posted to rec.radio.shortwave

dave

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Mountains don't simply block HF

On 02/13/2011 12:50 PM, wrote:
On Feb 13, 3:26 pm, wrote:
On Feb 13, 12:09 pm, wrote:

The signals don't travel; a field is set up around the transmitter
antenna, like turning on a lamp. When the field encounters an
obstruction it "knife-edges" and Fresnel zones are created, which may
help or totally prevent reception, depending on the math (distance from
transmitter to obstruction, from obstruction to receiver, blah blah). I
am in a box canyon, open only to the South. If mountains stopped the
signals completely, all I'd get would be penguins.

A sort of diffraction effect. Makes sense. - however, I would bet
some RF is blocked by either reflection or absorption, and the
probable chaotic diffraction from irregular edges likely results in a
pretty unpredictable pattern.

I don't quite picture this : mountains and canyons . Obviously-- no
line of sight signals such as local AM and FM broadcasts, very poor
groundwave propagation . Now, why is skywave affected as well ? What
can possibly block HF signals ?

The good skywaves come from 10 to 20 degrees up. The mountains are
higher than that. I get no VHF/UHF TV. I get FM from Mt. Wilson,
severely degraded. I get MW like gangbusters.

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