A fading-resistant method for RFID-antenna structural mode measurement

In this paper, a prototype device to measure the tag-antenna's structural mode term is proposed. The device comprises a switching transistor, controlled by a microcontroller, and a reverse-biased varactor diode connected to the transistor. The antenna's load is selectively modified by applying the appropriate DC voltage on the varactor. Antenna's structural mode is evaluated by measuring the backscattered modulated field for three states of the varactor. In contrast to prior art: i) the proposed device can be conveniently used to measure the structural mode of any antenna, via an RF connector, without the need to successively attach different loads to the antenna under test, ii) measurements can be performed under fading/changing conditions, as fading-effects are removed, as demonstrated herein. The device can be used to evaluate backscatter communications for different tag-loading. Furthermore, it could operate as an RF sensor, where the sensor's voltage “drives” the varactor's capacitance.