An analysis of surface/wire currents and charges of commercially available passive UHF RFID tag antennas

An analysis of surface currents and charges, as well as wire current variations in passive UHF RFID tags are presented to illuminate differences in tag antenna designs. Surface and wire current or charge distributions are known to be important parameters in distinguishing the radiation pattern of passive tags in the far field, and has been shown recently to have severe consequences on directivity patterns of passive UHF tags on products containing non-RF translucent materials. Commercially available passive tags are used to study the variations of these parameters, where a detail comparison is performed with the typical UHF RFID dipole tag antenna. Issues regarding segment currents are also introduced in order to initiate discussions regarding the physical variations of tags, such as deformities due to curvatures.