A low power RF harvester for a smart passive sensor tag with integrated antenna

The design and implementation of a low power RF harvester for use in smart passive sensor tags applied in structural health monitoring applications is presented. The tag operation is divided into two cycles: a standby mode cycle and a fully operational cycle. While in standby the tag requires only 5 μA at 2.5 V while any excess energy is stored in an accumulation capacitor and made available to the tag during full operation, where a low power commercial microcontroller responsible for sensor measurement and data transmission turns on. An optimized PIFA-type embedded antenna has been integrated in the design in order to enable tag operation near ground planes and metal structures. The system operates in the 430 MHz band requiring a measured minimum input RF power of −5.8 dBm. Tag operation has been tested by activating the onboard microcontroller. Indoor measurements as well as measurements with the use of a signal generator indicate full operation of the sensor tag for time intervals of 20 ms for up to 4 m distances with tag reader power of only 200 mW.