Analysis of Read Range for UHF Passive RFID Tags in Close Proximity with Dynamic Impedance Measurement of Tag ICs

In this paper, we discuss the change of read range between a UHF passive RFID tag and a reader when a tag is close to another tag. The read range is determined by the strength of the forward and reverse link. These links are identified by comparing the calculated received and reflected power with the measured dependence of the read range on the distance between close tags. We developed a system to measure the tag IC impedance in operation. The accuracy is assured by the short-open-load (SOL) one port calibration. The reader and tag's change of received power is calculated according to the change of tag IC impedance due to the closeness of the tags. As a result, it becomes clear that the read range is determined by the reverse link finally.