Hi Phil,
QFH is effectively circularly polarized, it is often used for GPS receivers, since when you point it skywards the main lobe points trough the zenith, while nulls are pointing to the horizon (where you do not have satellites normally).
In my opinion QFH risks to be quite bulky for a balloon, since it will occupy a cylinder 1 wavelength high (70cm in your case).
To keep it simple, I would rather go for a pair of crossed dipoles 90° phased over a ground plane: the crossed dipoles will be fairly larges (about half wavelength, 35cm) but light since it is made of some sort of rigid wire. The reflector can be the bottom side of the payload, if the latter is cubic-shaped and it's at least 35x35cm (otherwise, you can use a metal net and fix it to the bottom of the payload.
The crossed dipoles will be 1/4 of wavelength spaced from the reflector, that is 17.5cm in free space, but can be reduced if you insert some sort of dielectric (like Styrofoam).
The gain will be essentially an hemisphere, if you need to reduce it on the horizontal direction, just enlarge a bit the reflector, and bend it slightly trough the dipoles.
Unfortunately I do not have any figures on hand for the cross-polarization performances, but IMHO, this is the best complexity-performance compromise.
73s
Simone - IU1AFY (ex. IW1FYV)
2013/7/17 Phil Karn karn@ka9q.net
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.
Thanks,
Phil
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