On Wed, 19 Sep 2012 10:12:09 -0400, "Ralph Mowery"
wrote:

Do you think we have discovered one way shingles ???

Nope. You've discovered Fresnel zone diffraction, multipath
cancellation, and frequency selective fading. Congratulations.

Anyone else seen anything like this ?

Not on 6 meters, put plenty at 900MHz and up. Basically, if you have
more than one path between endpoints, and the path lengths differ by
1/2 wavelength, you'll get cancellation. It's easy enough to detect
the problem by simply moving 1/2 wavelength toward or away from the
other endpoint and see if the signal levels change significantly. If
you can't do that, change frequency by the largest amount possible.

Also, if you live 15 miles apart, with antennas at about 15ft off the
ground, you're below the horizon. That means you're relying on knife
edge diffraction to communicate. That works, but is not reliable as
almost any metal object near the knife edge can send the signal off
into the wrong direction. At 6 meters, things like swaying wet trees
can do that. If the signals are NOT stable and tend to move around in
the wind, this might be the problem.

One way paths are possible. It's not the path. It's the antennas.
The basic assumption is that the antenna pattern is the same in xmit
and receive. That's a fair working assumption but not always true.
Different side lobes of an antenna can have a slightly different phase
shifts. If the two multipath signals arrive via the main lobe and a
side lobe in receive, and the side lobe moves slightly in transmit,
it's possible to achieve cancellation only in one direction.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558