[amsat-bb] Help wanted on CP antenna design

Phil Karn karn at ka9q.net
Tue Jul 16 22:41:08 PDT 2013

I'm one of several adult mentors to a local high school ham club that's
been designing and flying balloons. We are designing a high speed 70cm
digital downlink for a future payload. One of my major concerns is
frequency selective multipath fading in the 1 MHz bandwidth I plan to
use as the balloon gets low on the horizon of the tracking station and
the receiver begins to pick up a reflection off the ground.

I think one of the simpler ways to deal with multipath is to use
circular polarization on both ends. This automatically rejects all
odd-order reflections (including the all-important 1st order reflection
off the ground) as they have the opposite circularity sense to the
direct signal and automatically be cancelled at the receiver. (CP was
tried experimentally with analog TV broadcasting back in the 1970s as a
means of reducing ghosting.)

On the ground we'll just use a regular 70cm satellite antenna, but on
the balloon I need a 70cm circularly polarized transmit antenna with a
fairly wide (hemispherical) beamwidth that can maintain its circularity
over as much of that as possible.

I'm not an antenna guy, but my understanding tells me that a quadrifilar
might be a good choice here. Any comments on this? Can anybody point me
to some typical performance figures for these antennas? Again, I'm more
interested in cross-polarization rejection than in absolute gain. I have
plenty of link margin and I'm willing to sacrifice a few dB if necessary
to get rid of those deep multipath fades.



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