Just a minor geeky point with apologies, but the 'cavity tube' used in older transponders like KT-76a is a travelling wave tube (a sort of microwave amplifier) rather than the magnetron as used in radars and given to the Americans during WWII.
As a kid I remember trying quite hard to obtain a magnetron from govt surplus shops for my home made radar project. Now every tip has a great pile of them in the form of microwave ovens!