Signal strength readout and miniaturised antenna for metal-mountable UHF RFID threshold temperature sensor tag

A new method for the wireless monitoring of a battery-free threshold temperature sensor based on a passive ultra-high-frequency radio-frequency identification (RFID) tag integrated with a bimetal thermostat switch is presented. The strategic placement of the switch brings about a strong modification in the tag antenna impedance as the temperature crosses over a threshold value. Optimisation of the narrow-band tag antenna to operate at the edges of the 902-928 MHz frequency-hopping spread spectrum RFID in the different switch states provides separate sub-bands for the readout above and below a threshold temperature. The results show that this enables an unambiguous sensor readout by monitoring the backscattered signal strength.