I recently linked a YouTube video describing an application I created to
try to measure incoming elevation arrival angles using the measured
phase difference between two antennas spaced some distance apart. The
application is functionally accurate, but the results I got were pretty
disappointing because the measured phase difference and subsequent
arrival angle display was heavily affected by what I believe to be
multipath effects from propagation, ground reflections, and nearby
terrain. When things were stable with no obvious propagation-related
multipath effects the displayed phase was stable but it still seemed to
be affected in magnitude by what I believe were ground reflections.
I have created an illustration of what I believe happens under multipath
conditions. The red and green waves in the background (you have to look
kind of closely) represent two incoming waves ... same signal but
arriving at slightly different angles, with both their angles and their
relative phase slowly changing over time. The brighter interference
pattern in the foreground includes the effect of ground wave reflections
and represents the combination of the four waves ... both of the
incoming waves and both of the ground reflections.
The is basically a 2D representation of different arrival angles and
relative phase. The real world would be 3D and look even more complex
over time.
http://www.ab7e.com/Interference%20Pattern.svg
The peaks and nulls in the combined wave don't simply represent
variations in amplitude. They also represent variations in phase, and
those phase variations can be extreme.
The interference pattern is the result of actual calculations (built by
Codex), but of course it uses arbitrary values. An actual situation
would likely vary significantly, but I think the illustration is
informative. At least it was for me.
73,
Dave AB7E
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