Earlier this year there was mentions that a manufactures (with help from NATS)were developing a handheld Transponder.
And they were looking for the end of the year for approval....
The device would be about the size of a large GPS.
A price of appox £ 500 was mentioned.
Kinetics tried to develope the LAST device a few years back.
Basically the same a hand held transponder but the CAA would,nt approve it.
Now its up to EASA to approve...
http://www.chinaga.com/bank/4499.pdf
ACS Avionics