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This paper presents the development of passive high frequency (HF) and ultrahigh frequency (UHF) radio frequency identification (RFID) sensor transponders for humidity and temperature monitoring. The transponders use RFID integrated circuits with a digital interface for communicating with a ultralow-power microprocessor. The microprocessor handles the measurement, either by charging sensor and reference capacitors, or by commanding a digital sensor. The read ranges of the sensor nodes are 5 cm and up to 40 cm at HF and UHF, respectively. The sensor nodes are demonstrated in concrete casts, where the HF node provides better than 5%RH-repeatability of the moisture measurements from sensor node to another. At UHF the repeatability is about 15%RH.

This paper describes a prototype of a UHF RFID based log marking and tracking system developed for the challenging four-season outdoor conditions in Scandinavia. The RFID system comprises of novel pulping compatible EPC Class 1 Generation 2 transponders, and of robust readers with novel performance boosting features. A wedge-shaped transponder is inserted into the log end with a special tool so that it is protected and held firmly in place by wood during the transportation and processing of timber. A robust EPC-compliant RFID reader featuring an adaptive RF front end was developed for use in a harvesting machine. Readability tests at saw mills with test logs using specially adapted commercial UHF readers show nearly a 100% readability for the transponders inside fresh moist logs. (14 refs.)

A wideband adaptive isolator for a UHF RFID reader with a minimum component count is introduced. The operation of the device is based on a reflection from a variable load, which is connected to a port of a hybrid coupler. The other ports are connected to the receiver (RX), transmitter (TX) and antenna ports of the reader. The isolator provides an isolation of 50 dB from TX to RX at the 800 MHz-1 GHz band, covering the worldwide UHF RFID bands. The adaptive system ensures high isolation up to an antenna return loss of 6 dB, or reflection coefficient of 0.5. The measured loss in the TX-to-antenna and antenna-to-RX paths is 4 dB.