I'd like to put a WSPR or JT65-like transmitter on a high-altitude balloon, but lacking much experience with HF antennas, I don't quite know where to start. I'm thinking it will operate on 20 meters, at (obviously) low power, say 100 mW or less. Building a lightweight little transmitter with an Arduino seems do-able, and I can hang a wire from it as long as I like, within reason, but I'm not sure how to match to it, what to use for a counterpoise, and so on. Radials longer than about a foot are out, for example.
I've googled a bit, and found all kinds of stuff about HF antennas lofted by balloon, but all those are tethered to the Earth. This will be free-floating. No ground plane to be found anywhere, no transmission line (if I can help it), no big structure holding wires apart. Weight is a big issue.
Edit: another concern. The problem with a dipole occurs on the way down, after the balloon bursts at altitude. Things go thrashing around pretty badly at that point, until the atmosphere gets thicker and the parachute can take hold. I have read at least one claim that the whole package can almost go supersonic!
So I worry about what a weight at the end of a 5 meter wire might do. Especially since this won't be the only payload on this balloon. I can just imagine what a 5 meter wire weighted at both ends could do, which is what I'd be looking at if I hung the electronics package in the middle of a dipole. Far better to put the weight as close as possible to the parachute, in one place if I can do it.
Maybe I worry too much. I thought about a simple quarter-wave wire, but then what's the counterpoise? There has to be something.