I searched for a long time online but was not able to find a lot of information regarding how successful people were using the xbee pro xsc radios at 20 miles up.  I did read one report of a balloon that used a 2.4 ghx xbee @ 60 mW.  It got to 10000 meters (one thrisd the distance we need to go) before losing signal.  However we are using 900mhz vs. 2.4 ghz and 100mW vs. 60 mW.  So the question is how much does this change the results?  The extra power certainly will help, and out path losss will be less at 900 mhz.  I decided we need to build the best antenna system we can to ensure the best performance out of the xbee system.  In other words, the antenna setup in NOT trival.

Years ago I was into putting cameras on r/c aircraft and sending them up to fly around and send live video.  The equipment was pretty expensive and we were always try to get the best performance out of the least amount of power.  I had an issue at that time where banking the aircraft left or right cause the video signal to fade.  It was then that I learned about circular polarization.  I even have 2 helictical antennas left over from those project, but unfortunately they are made for a slightly different frequency.  So I decided to check out the r/c forums to see if anyone had caught on the using circular polarization.

Other than being fascinated by the leaps and bounds that the ‘FPV’ hobby has made in the last 5 years or so I found a wealth of information about building many types of circularly polarized antennas, most from the member IBCrazy.  I have to say, he provides really great tutorials on the antenna construction.  I also found this site which is a really excellent visualization on antenna polarization.

Anyway, here is a video of the antenna that will be balloon-side:

And the helictical antenna we plan to use on the ground (or in the SAR plane – more on this later).