The present invention relates to a transponder comprising an antenna structure (31) configured to receive an incoming signal with RF power of a first frequency f1 and to retransmit the incoming signal as a transparent output signal with RF power of a second frequency f2 when hit by the incoming signal. The transponder further comprises: an amplifier (39) configured to provide an amplified transparent output signal based on the incoming signal; a frequency converter (35) configured to frequency convert the incoming signal into the transparent output signal; and a first frequency filter (33) connected to the antenna structure. The first frequency filter is tuned to the first frequency f1 to avoid amplifying incoming signals with RF power of frequencies other than the first frequency f1. The transponder further comprises a sensor (S) configured to respond to physical stimulus for generating a control signal as output response, and an activator for selectively activating or deactivating the possibility to retransmit based on the control signal of the sensor.