I mentioned in an earlier post that I was investigating ways to measure
and plot incoming signal arrival angles as a function of time. I just
think it would be interesting. but there is probably some practical
value considering that many antennas have a notch in the elevation
pattern depending upon height above ground, and if the signal meanders
through that notch we'd get a lot of fading.
I have an Elecraft K3 with two receivers that can be phase locked, and
since phase is preserved through down conversion I can feed the stereo
Line Out audio from the rig into the stereo Line IN port of my computer
sound card and capture the difference in phase for an arriving signal
incident on two antennas spaced some distance apart. Using Codez AI I
built a browser app that does just that process and it works fine. I
can feed two RF signals into my K3 from a 2-channel RF generator with
controllable phase shift, and the displayed phase difference for the
audio in the browser app is right on target. The app gives me a
scrolling plot of the phase difference ... both "instantaneous" and
smoothed. There is a calibration slider to adjust for phase differences
in the feedlines, the receivers (the K3 receivers are phase locked but
the absolute phase changes at turn on and whenever the crystal filter
changes), and potentially the sound card channels. Sorry I can't
include a screenshot here, but I can send one of an interim version of
the app if you're interested.
However, when I tried some real life experiments using the two yagis on
my tower (a 2 element 40m and a tribander) I got very squirrely
results. The displayed phase shifts were WAY greater than could be
explained by the frequency and spacing of the antennas and they varied
quite rapidly in time. I was using WWV at 10 MHz for the signal source
with my K3 cranked down to a 200 Hz bandwidth to mostly reject the AM
sidebands. I suspect that the parasitic nature of the yagis exaggerates
the phase response for varying arrival angles (especially for off-axis
signals) since parasitic antennas work by manipulating phase, but even
when I used EZNEC to model the phase response of two simple dipoles one
above the other the phase difference between them didn't match the
theoretical sin() profile at all.
I suspected that ground reflections were the culprit so I first tried
modeling vertical dipoles one over the other since vertical polarization
is less affected by ground reflections, and while that was a little
better it didn't fix the problem. However, the one thing that DOES work
in the model is to use two horizontal dipoles at the same height above
ground spaced some distance apart horizontally since at least
theoretically the ground affects both antennas equally at all arrival
angles. That model tracks almost perfectly with the theoretical cos()
profile we would expect ... like to a small fraction of a percent. The
problem with that configuration, though, is that the signal needs to be
directly broadside to the dipoles for the calculated angle to be valid.
There are ways to solve all of that if you use enough antennas and
probably multiple receivers, but one simpler(?) possibility might be to
put two horizontal dipoles at the ends of a roughly 20 foot boom in
addition to two vertical dipoles also at the ends of the boom. The
antennas don't need to be resonant, and in fact you don't want them to
be resonant so the shorter the better as long as the resultant signal is
strong enough to register cleanly in the app (or whatever else you use
to compare phase). Use a pair of DPDT switches at the antennas to
switch between the horizontal and vertical dipoles to feed two runs of
coax (properly choked) back to the shack. Use the vertical dipoles to
get a phase difference of zero in the app for the desired signal when
rotating the boom, then rotate the boom 90 degrees and switch to the
horizontal dipoles to measure/plot the arrival angle. A small
programmable signal source like an inexpensive Si5351 module located at
the center of the boom could be used for calibration.
I don't know if I will actually build an antenna system like that or
not, but I will finish up the app and make it freely available for
anyone who wants to try something similar. At the very least it could
be used with two ground plane verticals (or three verticals if switched
to eliminate the ambiguity inherent with just two antennas)) to
determine azimuth, but it still would require two phase locked
receivers. A possible non-transceiver alternative that should work is
the SDRPlay RSPduo that sells for about $300 and has two separate
receivers that can be phase locked. I think it could be used with the
SDRuno app (free) to provide the required audio feeds to the app using
the VB-Audio virtual audio cables app (free for the 2-channel version)
although I haven't personally tried it.
Sorry for the long post, but I wanted to summarize things for the folks
who had asked. Comments are welcome.
73,
Dave AB7E
p.s. In case anyone is interested, I used a third dipole in the model
as a 10 MHz source for the incoming wave. I positioned it 10,000 feet
from the two dipoles and changed it's position in an arc that maintained
a 10,000 foot distance for all measurements. As an example, 45 degrees
is 7070 feet for the X dimension in the model and 7070 feet for the Z
dimension. The length of the two dipoles had negligible affect on the
results as long as it was short enough (roughly 20 feet or less in the
case of 10 MHz).
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