An RFID tag (1) is configured to transmit a predetermined code (3K) as a RF backscattered radiation (2) in response to an impinging RF signal (41). The RFID tag (1) is configured to react to an impinging signal at a predetermined reference frequency (23) with a reference backscattered signal (2R). The RFID tag (1) is also configured and to react to an impinging signal (41) at any of a group of transmission frequencies (21, 22) with coding backscattered signals (2F-G) whose amplitudes (20A, 20B) relative to the amplitude (20R) of the reference backscattered signal define the code (3K). An RFID reader (4), a kit (5) and a method for transmitting a message from a device (1) to a reader (4) as a RF backscattered radiation (2) in response to an impinging RF signal (41).