Radio frequency signal modulation by impedance variation

A method for transmitting a code comprising a series of juxtaposed symbols from a device to a reader as a RF backscattered radiation comprising:

• providing an RFID tag device configured to transmit RF backscattered radiation in response to an impinging RF signal,
• emitting with the reader a reference interrogation RF signal at a reference frequency to the RFID tag,
• receiving a reference backscattered signal at the reference frequency from the RFID tag in the reader,
• emitting with the reader at least a transmission signal at one trans mission frequency to the RFID tag, the transmission frequency being different from the reference frequency,
• receiving at least a coding backscattered signal at the transmission frequency from the RFID tag in the reader,
• comparing an amplitude of the reference backscattered signal with an amplitude of the coding backscattered signal,
• defining a symbol based on whether the amplitude of the reference backscattered signal is greater, equal, or smaller than the amplitude of the coding backscattered signal,
• wherein the symbol represents more than one digital bit.