Hi, Jim.
The orientation used in the video was two horizontal dipoles both 18
feet above "ground" (the surface of the earth, not necessarily the true
RF ground plane) spaced 12 feet apart and positioned broadside to WWV.
The frequency for the drone flyover was 28.26 MHz and the WWV frequency
was 15 MHz (30 meters). Clearly the spacing was better at 28 MHz but
the result was still massively affected by ground reflections.
But you were correct in your earlier post that antennas stacked
vertically will not give accurate results to them having different
ground effects. Both AC6LA and I proved that several weeks ago using
EZNEC, with that discussion being located on the EZNEC groups.io forum.
Like you, I also thought that maybe vertical antennas would be less
affected, but EZNEC told me that over/under stacked verticals were just
as affected by ground reflections.
I did, however, rotate my horizontal dipoles to be vertical just to test
azimuth, but since an arriving signal not perpendicular to the plane of
the two antennas has a phase determined by both its elevation and
azimuth that wasn't useful either.
In any case I have improved the interference pattern visualization. The
SVG version had some limitations and some errors, so just a few minutes
ago I posted a link to a much better HTML version.
http://www.ab7e.com/Interference Pattern.html
In that post I made this point .... what we measure from two antennas is
the gradient of the interference pattern caused by multiple paths. And
there will always at least be multiple paths caused by ground
reflections even if propagation is stable and giving us only one signal.
I have pretty much convinced myself that what I was trying to do is
futile because the results I got with my ArrivalAngle application and
test setup correspond almost exactly to what I would expect if all I was
measuring was a diffraction gradient.
73,
Dave AB7E
On 4/14/2026 12:45 PM, Jim Lux wrote:
what was the relative positions of the two antennas (ht above ground?), and the wavelength
On Mon, 13 Apr 2026 23:05:07 -0700, David Gilbert via TowerTalk
<towertalk@contesting.com> wrote:
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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