Zero-power, long-range, ultra low-cost harmonic wireless sensors for massively distributed monitoring of cracked walls

A novel zero-power wireless crack sensor based on the harmonic radar principle is presented. The tag, fabricated on a paper substrate by means of the copper adhesive tape technology, is targeted for a fundamental frequency f₀=2.45 GHz (ISM band) and consists of a system of two nested annular slots, a frequency doubler and a stub behaving as a band-stop filter. In presence of a crack, the stub, placed at the input of the doubler, is torn off and an alarm is sent to the receiver. Such a system is suitable for scenarios which involve the detection of any crack increase in a massively distributed population of cracked wall sensors. A wireless experiment demonstrates an operating range of the sensor from 1 to 5 m for a transmitted power EIRP of 25 dBm.