I am going to send up a few micro beacons (milliwatt HF CW transmitters) probably using Walmart balloons and thin wire as the active antenna/payload string.
The CW beacon RF module is a really simple oscillator that drifts a certain amount with temperature - that way I can tell how cold it is based on the received frequency from on the ground. An ATtiny or something similar will probably be used to keep the beacon legal and send out CW every 10 mins to break the carrier.
So the whole RF module and beacon is extremely light, basic transistor/crystal oscillator + ID chip so I don't have to worry about recovery. The only consideration is the battery. I will probably have something operational between 3 and 7 volts, so I am thinking about just stringing a few watch batteries together. Is there a certain recommendation for low temperature operation miniature batteries? I need to make this payload as light as possible.
The goal is the cheapest Amateur Radio balloon project where recovery is not a worry. Signal strength reports across the country as well as wind predictions can get a good idea of where the balloon travelled. I can then match this data up with the daily National Weather Service launch data to determine how high my balloon went based on temperature.
73 Skyler KD0WHB