Microstrip antennas for remote moisture sensing using passive RFID

The paper presents how materials with a high ability of absorbing moisture can be used with microstrip antennas and passive RFID chips for constructing remotely read moisture sensors. The concept is based on pairs of microstrip antennas, applied on the same piece of PCB and each equipped with one RFID chip, where one antenna is covered by the moisture absorbent material and the other is left open. In a humid or wet environment the moisture concentration is higher in the absorbent material than in the surrounding environment which causes degradation to the embedded antenna both in terms of dielectric losses and change of input impedance. The level of relative humidity or the amount of water in the absorbent material is determined for a passive RFID system by comparing the difference in RFID reader output power required to power up respectively the open and embedded tag. While it has previously been shown how this can be accomplished with simple one-layer antennas we here look into using microstrip antennas to get around the problem with high influence from background material. Typical applications include moisture detection in buildings, especially from leaking water pipe connections hidden beyond walls or simply to measure the level of relative humidity at hidden locations.