[CQ-Contest] Is HF HF (sic) propagation reciprocal?
Ethan Miller
k8gu.ethan at gmail.com
Wed Jun 6 15:29:40 EDT 2007
Zack ratted me out. First of all, allow me to say that although I
spend my days (and more than a few nights) studying the burps and
warbles of the ionosphere, I am probably little more of an "expert" on
HF propagation than anyone else on the reflector. This does deviate
somewhat from the topic of contesting; but, inquiring minds want to
know!
A few additional comments (some of which have been raised by W6WRT and
K0XP): I am not familiar with the modeling package used; so, I don't
know how detailed its ionosphere model is. Furthermore, SNR does not
technically a measure of reciprocity constitute (in the classical
sense.) See an introductory linear systems or electromagnetics text
for details. That said, the SNR effect described is well-known by
DXers and contesters alike. The article that started this thread is
an accessible worked example of some special cases.
The reciprocity of skywave links is a very old topic indeed. The
underlying principle we need is Goubou's theorem (1942). I now reach
for a 2007 Hamvention flea market treasure (tnx N8TR and N8DMM):
"Radio Waves in the Ionosphere: The Mathematical Theory of the
Reflection of Radio Waves from Stratified Ionised Layers" by K. G.
Budden (Cambridge 1966). According to Budden, Goubou's theorem may be
concisely stated the following way: "For any aerial system at A it is
always possible to construct at least one aerial system at B so that
there is reciprocity between the terminals of the two aerials." In
other words, reciprocity between two points is always possible, but
not guaranteed for any given pair of antennas at those points. Budden
continues with the proof and a discussion including magnetoionic
effects. I invite you to look it up if you're really interested in
this.
I think our collective experience bears the theory out in practice.
That is, many practical amateur antenna systems have a high
probability of reciprocity. However, we do get surprises now and
then.
Regarding the tilt of the ionosphere: this is most important on paths
where the terminator is involved. It may be a sufficient, but not
necessary, condition for nonreciprocity. I'll refrain from making any
more comments on the physics because I haven't spent enough time
thinking about it.
See you in the mayhem...I mean on the air, not just the reflector...
73,
--Ethan, K8GU/9.
http://www.k8gu.com/
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