Hi LF folks,
John wrote in "LF: Weekend report" on 06/03/00
The local variation in signal strength was again very apparent on Sunday
morning when HB9DCE's signal was poor with both G3YXM and myself, but
Berth was getting R5 reports from GB2CPM and G3XDV - the reverse of the
situation Mike reported last time. It would be very interesting to establish
for certain whether this is due to an enhancement effect (i.e. Mike and
Peter were receiving better than other stations either side) or a
cancellation effect (i.e. Dave and I were in a null). Perhaps we could set
up a 'reports pool' for noting simultaneously the strengths of stations in
Switzerland and Italy (whose signals seem to produce this effect over the
UK) by a number of observers?
Regards John G4GVC near Leicester, IO92JP
Thinking about these observations, there is a striking analogy to the speckle
patterns seen in a number of fields, e.g. atmospheric seeing in astronomy,
ultrasonic tissue imaging, ground clutter in microwave radar, or (most well
known) the structure in a laser spot on the wall. Basically it is caused by
the superposition of a number of coherent wavefronts generated by stochastic
scattering from some inhomogeneous medium. For LF, flat terrain with trees,
houses etc. is smooth because all the obstacles are small compared to a
wavelength. Mountain ranges however are a rough surface. The angular
granularity ("directivity") of the resulting pattern depends inversely on the
lateral width of the radiating zone. If one would assume that Toni's rays are
diffused over a region say 20 km (20*lambda/2) wide, this would result in
lobes 1/20 radians wide or 50 km "coherence width" at 1000 km. It would be
interesting to derive that by observations, I'd expect a somewhat smaller
coherence width for broader illumination from transmitters behind the Alps.
As the modulation is spatial, not temporal, speckle patterns normaly are
static, unless something moves. As the scattering centers (mountains!) are
fixed, we might imagine the ionosphere acting like a slowly tilting mirror,
sweeping the whole pattern across the receivers.
But maybe the scattering centers are not only the rocks themselves: Bob
Brown, NM7M, had a fascinating article in QST 12/99 "Bubbles in the Ozone
Layer", suggesting that turbulent air-flow over mountain ranges could cause
bubbles affecting the ultraviolet irradiation to the D-Layer before sunrise.
Regards
Markus, DF6NM
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