rf tx that operates in uA?

Can somebody recomend a tx module that draws uA @ 3V? I want to make a transmitter that has a range no more than 5 feet and draws uA.

Describe what problem you are trying to solve.

Presumably you want to transmitt data?
What bit rate?

There are wireless doorbell transmitters that operate from a 3V coin cell.

If you just want on/off signalling you might be able to use an RFID tag with a switch that turns it on or off, inductivley powered from the receiver - no battery.

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 .