Impedance Properties and Radiation Efficiency of Electrically Small Double and Triple Split-Ring Antennas for UHF RFID Applications

This letter is targeted at investigation of electrically small planar antennas, formed by double and triple split rings that are intended to serve as tag antennas operated within the European UHF RFID band. An extensive systematic study of possible geometrical configurations demonstrates the range of their achievable input impedances (complex conjugate to typical UHF RFID integrated circuits) in the case that both structures electrically decrease below the limit for electrically small antennas, i.e., ka <; 0.5. The advantage of the triple split-ring version over the standard double split-ring one consists in reduction in its electrical size and concurrent maintenance of the required range of input impedances and high radiation efficiency. Samples of both structures were manufactured and measured, and their electrical properties were critically compared.