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Re: (meteorobs) radio meteor polarisation diversity
Hi Bruce,
I'll reply to your last message as soon as I can, but I have been taking
care of my father who has been ill and who had to go to the hospital on
Monday--he's back home now. Now I'm under the gun because I'm trying to
prepare for a presentation (actually on meteor scatter communications) for
Saturday morning and I have to have a math article written by Saturday as
well.
However, not all is negative in these situations because while I was
looking through my reference materials for my presentation on Saturday I
found this:
D-layer absorption often is neglected in link budget analyses of meteor
burst links. ... D-layer absorption, a daytime-only phenomenon, may exceed
3 dB at 40 MHz at midlatitudes during comparatively high sunspot number
periods. It is about half this value at 60 MHz.
Yet another path loss is due to Faraday rotation. The D region and Earth's
magnetic field cause a linearly polarised VHF wave to be rotated both
before and after meteor trail reflection. These rotations result in an
overall end-to-end loss due to polarisation mismatching between an incident
wave and a linearly polarised receiving antenna. An excellent paper dealing
with Faraday rotation effects on meteor burst communication links was
published by Cannon in 1985. [P.S. Cannon, Polarization Rotation in Meteor
Burst Communication Systems, Royal Aircraft Establishment Tech. Report
85082 (TR85082), London, Sept. 1985.]
As in the case of D-layer absorption, polarisation rotation loss is
affected by path length (secant of the take-off angle), the Sun's zenith
angle, and the sunspot number. Like D-layer absorption, Faraday rotation
losses also disappear at night. Faraday rotation losses decrease rapidly as
frequency increases. Cannon shows Faraday rotation losses varying from 1 dB
to over 15 dB at 40 MHz (very path dependent) and dropping to a maximum of
1.6 dB at 60 MHz on the same paths.
I included the information about Cannon's report in case you have access to
it.
Given this information, it hardly seems worth even using cross-polarised
antennas let alone a helix. I guess this explains why we didn't notice any
improvement when we tried using the cross-polarised pair of yagis. I guess
I should have read my references more carefully!!
Phil
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