[TowerTalk] Arriving signal interference pattern

[David Gilbert]

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