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.

Size - in order to work effectively each side of the corner must be at least 1.5 wavelengths - this gives a size of about 15cm to cover down to S band RADAR (3GHz = 10cm wavelength) - this makes a suitable reflector about 30cm (1 foot) 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).

Can anyone confirm that aircraft radar (airborne and ground based) is S band or higher?

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.

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.