Backscatter radio frequency (RF) tags and radio frequency identification (RFID) tags can be used to create wireless sensors that do not require a battery by using the tag antenna’s electrical load as a transducer. This paper presents such a backscatter sensor system operating in the 5.8 GHz unlicensed frequency band that wirelessly senses displacement between one and five millimeters. The system senses the variable impedance of a covered, open-circuit microstrip line and uses two fixed antenna load impedances to remove the effects of the radio channel. The primary contribution of this paper is a demonstration of the wireless sensing system in which the binary position of a human finger (e.g., “bent” or “straight”) is measured using a passive transducer for wireless human-computer-interaction (HCI) applications.
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