Hi Phil, I've flown a couple of balloons as an advisor to an undergraduate lab here at Virginia Tech. We use CP on all of our flights, though generally we use the standard 2m APRS freq and not 70cm. Our first two flights we flew a crossed dipole suspended above the payload (to get it out of the view of the down facing camera) and had excellent results. Our second flight flew the Byonics MicroTrak 300, with only 300 mW of power and had no problems hitting digipeaters and iGates from 100,000+ feet. The crossed dipole was constructed out of aluminum arrow shafts (rigid, but lightweight). In an effort to drop even more weight, we changed to a design that uses fairly rigid wire in a "turnstiled loop" configuration, more commonly known as an eggbeater, again with excellent results from flight. In both cases we decided to forego installing the ground plane mentioned in previous posts. This was primarily a design decision made to save weight at the loss of a few dB (energy radiated up does us no good with receivers on the ground). One design that I have wanted to try but haven't yet had the opportunity is the crossed moxon. Essentially, it is like taking the reflector and driven elements of a pair of yagis and crossing them at 90 degrees. It is slightly different from the Yagis as the end of the reflector is bent 90 degrees toward the driven and the driven ends are bent 90 degrees towards the reflector (for impedance manipulation). L.B. Cebik, W4RNL, wrote a very useful article in the August 2001 edition of QST describing the construction of the crossed moxon, complete with design dimensions for a 435.6MHz crossed moxon (google can direct you to a copy of his article). I do not have information about the cross-pole rejection, but some time spent with 4NEC2 antenna modelling software may give you a bit of insight. With the bends in the elements of the moxon it focuses the pattern a bit more towards the horizon (over the crossed dipoles). I'm not sure about your location, but for us in SW Virginia our balloons get whipped right along in the jet stream and we have trouble keeping up with them during the chase (peak lateral speed in the Jetstream for our flights was 143 MPH). The result of this is that at no time (other than the initial release when the balloon is under about 10,000 ft) does the balloon get much higher than maybe 20 degrees elevation. Having energy focused more towards the horizon and less towards zenith (both in the balloon antenna and the ground antennas) is probably not a bad Idea in our case.
If you search callsigns KK4MOB and KK4PWM on aprs.fi you can see the path of our flights and the insanely high speeds in the Jetstream (around 30k-40k ft).
Hope the responses help inform your decision. GOOD LUCK!
-Zach, KJ4QLP
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Zilvinas, LY2SS Sent: Wednesday, July 17, 2013 6:33 AM Cc: AMSAT BB Subject: [amsat-bb] Re: Help wanted on CP antenna design
On 2013.07.17 10:11, Simone wrote:
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).
QFH for 70cm band is 264 mm height only (and 84 mm in diameter).
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