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Re: polarity reversal at low elevations



 Estes Wayne-W10191 writes:

> I think Richard Limebear wrote
guilty

>> Go for RHCP; The only thing you'll lose out on is that, at  
>> low elevation (say below > 10 degrees), signals get scintillated to the 
>> opposite sense so an RHCP signal would get turned round ... but its not 
>> usually by a complete turn.
> 
> Interesting.  I never knew about polarity reversal at low elevations.  Is 
that cause by ground reflection? 

No. Its a propagation effect. Something to do with ionospheric layers (well, 
something a long way up anyway). At the shallower angle of a low-elevation 
signal the signals have a longer path through the layers and get hit more
than higher angle signals. Scintillation should be in the index of most
reference books (its in my RSGB handbook, probably in the ARRL one too).

I used to notice this often when I worked the digisats: went for LHCP at
AOS (wouldn't work on RHCP) then, once the elevation rose some, I switched 
to RHCP for the rest of the pass. Can't remember if it hit the LOS end of
a pass. Also often noticed it on AO-10/13; always worth a quick flip of 
the polarization switch just to see what happens (both on uplink *and* 
downlink, not always the same effect on different bands).

73
Richard W L Limebear G3RWL
g3rwl@amsat.org
FOC # 1188

          So many beautiful girls ..... (sob) so little time
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