Russian bug in the American embassy
In researching an article about wireless networking I happened upon one of the earliest uses of wireless machine-to-machine (M2M) applications. The Russians put a bug in a large wooden coat of arms that was given to the American embassy in 1945. The cool thing about this bug is that it was un-powered. It was the first RFID device in that sense. But whereas RFID devices work by rectifying the microwaves beamed over them to power a little CMOS chip and radiate data back, this passive bug just modulates the backscattered RF energy. The linked article did not really explain how the RF was modulated so I asked RF consultant James Long. He writes:
The device is an antenna. It scatters (reradiates) signals. The variable capacitor on one end changes the resonant frequency. This changes the phase of the scattered signal. The modulation is PM [phase modulation].