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Circular Polarity



Message text written by Ken Freedman
>
  I  could find no 
polarity info for any antenna on AO-40 except the 70 cm arrangement which 
is RHCP.  I did find an article by G3RUH about how to build an S-band dish 
(got that tucked away on the hard drive!) for AO-13, in which he uses a 
helix at the feed, wound "opposite to a normal screw thread".

      I believe that a signal that leaves the bird from an  RHCP antenna 
would want to see an LHCP antenna on the ground, unless a dish were in use 
which would revers

TNX and 73, Ken<
-------------

        ALL AO40 antennas are polarized RHCP (OK, maybe the HF whips
aren't), and the antenna on the ground must also be RHCP. G3RUH's dish feed
is wound LHCP because the signal is being reflected off of the dish, which
reverses the hand of circularity. 
        If the circularities are opposite you could theoretically incurr a
30db loss, though that would only be under perfect conditions. For the real
world, think of 2 intersecting elipses (from the never-perfectly circular
or never-perfectly aligned patterns of each antenna), so things may not be
exactly as the theory dictates. If in doubt, circular on one end and linear
on the other incurrs only a 3db loss (again, theoretical). And always
worked fine on AO10 and AO13. Your results may vary.

73,
Ed Krome K9EK
author/editor "Mode S:The Book", which, incidently, has all this kind of
stuff in in. Call Martha at AMSAT and order one!

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