I should have done more reading. I'm one of those guys who reads a
manual enough to make something work to my satisfaction, and then figure
I'll learn the rest of it with use.
I've read the pdf file written by N6BV that accompanies HFTA several
times through, but tonight I actually paid heed to his suggestion to
read the portion of Chapter 3 of the ARRL Antenna Book that discusses
HFTA and ray tracing in detail. I've scanned those seven pages of text
before, but prior to this evening I had never really read all of them
thoroughly. After a length explanation of how the combination of
reflections and diffractions from irregular terrain affect transmitted
signals, the second to last paragraph reads thusly:
"It is fascinating to reflect on the thought that received signals
coming down from the ionosphere to the receiver are having encounters
with the terrain, but from the opposite direction. It is not
surprising, given these kinds of interactions, that transmitting and
receiving might not be totally reciprocal."
It occurs to me that there might be a way to see if this actually
happens. A station in the U.S. could pair up with a station in Europe
to quickly exchange CW signals at regular intervals over an extended
period of time, possibly (shiver shudder shake) ala an automated AMTOR
mode. It would be best if one of the stations had flat terrain devoid
of significant features and the other one had one or two significant,
asymmetric features. The two stations could periodically (say once
every ten minutes) exchange a CW signal, transmitted at a consistent
amplitude and measured by stable receivers. The two stations would not
need to be equivalent ... only consistent ... and the parameter of
interest would be the ratio of the recorded signal strengths (received
signal at A divided by the received signal at B).
Over a period of time the takeoff and arrival angles for the path will
change. As I understand it, the angles for a DX path are typically
lower at the beginning and ending of an opening, and higher once that
path has fully opened. If transmitting and receiving are totally
reciprocal, and assuming that the ionosphere is essentially reciprocal,
the ratio of the received signals should be reasonably constant over
time no matter what the angle. If, on the other hand, asymmetric
terrain features contribute to non-reciprocity, that ratio is likely to
change as a function (probably not a linear one) of the takeoff/arrival
angles on the end of the path with the irregular terrain. We don't need
to actually know the angle, we just need it to be variable. My guess is
that, in general, higher angles would be more reciprocal and lower
angles might be less so, but after averaging out QSB (which would show
up as a time variable measurement for both directions) any significant
shift in the ratio would indicate some sort of non-reciprocity.
Finally, and for what it may be worth, the various test examples I ran
for myself in HFTA while I was trying to reason all of this through
suggested that higher antennas were less susceptible to the asymmetry of
terrain features than were lower antennas. Wouldn't ya know it?
73 to all,
Dave AB7E
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