On Feb 13, 3:26*pm, bpnjensen wrote:
On Feb 13, 12:09*pm, dave 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 ?