Responder Circuit

Hi,
I want to make a circuit which receives a prespecified frequency and returns back a response. Can any one help me..??
I want the circuit to be as tiny as 2X2 cm.
My idea was to put a an oscillator in fm range but i dont know how to incorporate a switch which recieves the frequency and switch on the oscillator.....

Can any exprienced guy give me suggestion on my this plan:-
ANTENNA--->AMPLIFIER--->FREQUENCY DOUBLER-->BACK TO ANTENNA

So when the circuit receives it doubles the frequency and retransmits back through the same antenna.................thanks

What range are you trying to get? This is sort of like what they put in library books. Those devices have a range of only a foot or two, and use a huge loop antenna for the interogator.

range is about 1.2 km (~.8mile).........