What about adding a Radar reflector to your balloon project - to make it more “visible” to aircraft?
A simple “corner” reflector is best - these produce an excellent radar echo because of their shape - the corners return the radar energy back in the direction it originated (the way “cats eyes” in the road work) - you will often see these used on boats.
FAR101 says “the reflector will present an echo to surface radar operating in the 200 MHz to 2700 MHz frequency range”.
Size - commercial radiosonde radar refelctors are about 50cm tip to tip.
A very lightweight reflector (less than 30g) can be constricted using carbon fiber kite spars and metalized plastic foil (as used in space blankets).
Pyrotechnic Cutdown Device
This device is used to detach the payload from the balloon (or release items attached to the payload). Its an igniter (used in amateur rocketry) inside a small hard plastic tube (sealed with epoxy at both ends) - when fired the igniter has enough explosive force to shatter the tube (and hence release the balloon) - it works a treat in all the tests I have done, including some at -30dec C and in a vacuum chamber.
WARNING: wear full eye area wrap around safety goggles when testing these devices.
Size: 45mm x 8mm diameter. Weight: a few grams. The igniter needs about 2amps @ 3Volts for just a few milli seconds to fire - it can even be fired from a charged 4700uF capacitor.
The ignitor on the right hand side of the circuit can be replaced by a small 6V bulb for testing - the bulb will light briefly when fired. Once you are happy with the circuit try an igniter.
adeptium/STARS Project: Not happy about the use of a wet cap for near-space use, but Farnell supplies an AVX 3300uF tantalum, 4-10V operating range, rated to -55C with no loss of capacity. I'm going to try with one or two of these.]