Radio frequency signal modulation by impedance variation

A RFID reader arranged to emit an interrogation RF signal, receive backscattered signals from an RFID tag, and reconstruct a code encoded in the backscattered signals,

• wherein the RFID reader is further arranged to:
• measure a reference amplitude of a backscattered signal at a predetermined reference frequency,
• measure the coding amplitudes of backscattered signals at any of a group of transmission frequencies, and
• determine a code of the RFID tag based on the coding amplitude relative to the reference amplitude.

A method for transmitting a code from a device a reader as a RF backscattered radiation in response to an impinging RF signal generated by the reader, the method comprising:

• providing a reference backscattered signal in reaction to an impinging signal at a predetermined reference frequency;
• and
• providing a coding backscattered signal in reaction to an impinging signal at a first transmission frequency;
• wherein the amplitude of the coding backscattered signal relative to the amplitude of the reference backscattered signal defines the code.