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The problem is tricky - he wants to have consumption about 3 microwatts .
So oscillator must be very efficient , that put restriction on using UHF, which is preferred for such communication - antenna lenght could be very small)
I think you can experiment with ferrite antennas on LF-HF band to keep power consumption low.
Or may be usltrasound will be the choice ?
And of course throwaway idea to use backscattering is good as well , if you dont have strict powerconsumption requirements on receiving side . But it wont be possible to use normal rfid because
if not wrong only microchip provided rfid for data aquisition but that is out of production now .
So you will need to design your own using zero bias detector diodes and mosfets to modulate back . I remember, rfid design for LF was posted here completely made on discretes wihtout using special IC's .