Could somebody please explain to me the concept of a "portable" transponder?
A transponder nowadays means almost everywhere a Mode-S XPDR. Part of the technology is the transmission of the registration of the aircraft, so "portable" does not make sense to me, as the current technology is bound to the registration and hence aircraft anyways. Yes, you can carry some, but why should you? Low power does make sense and many of the modern solid state ones are (i.e. a TT31 does pull only 0,5A when active, so very little batteries would drive that sufficient - 12V/2Ah is good for 4 hours and is what, 300 grams?), but portable?