Design of a Passive Retrodirective SHF-RFID Transponder for Communication and Localization

In this paper, we present the design, fabrication, and measurements of an antenna array that combines characteristics of retrodirective arrays with the direction-independent power receiving capability of rectenna arrays for use in passive SHF-RFID systems. This addresses the higher free-space path loss in the SHF band to enable passive transponder systems in this frequency domain with a range comparable to UHF-RFID systems. By using retrodirective arrays, the modulated backscattered power at the reader is increased without being limited by the strong directional dependence of a high gain transponder antenna. By combining this retrodirective array with a rectenna array the available power at the transponder is increased again without being constrained by the directional dependence of a high gain antenna. The presented 2 × 2 array was designed to achieve both increasing the modulated backscattered power at the reader by a factor of 16 and increasing the available power at the transponder by a factor of 4. In doing so, the directional dependency of the transponder does not increase. With the presented array, the range of the passive backscatter system can thus be doubled. This will enable future passive SHF-RFID systems with a range comparable to that of passive UHF-RFID systems. The higher bandwidth of the SHF band additionally enables a precise localization of the transponders. The higher free-space path loss can be counteracted using the arrays presented here.