Full-Sphere √ΔRCS Characterization of Passive UHF RFID Tagged Objects

Abstract

Characterizing the complex-valued response of passive UHF RFID tags over the full sphere is essential for any application that relies on the tag’s phase information, yet no suitable measurement methodology has been reported. This paper develops a measurement system that determines the tag’s direction-dependent complex response across the entire sphere. We propose a model based on the square root delta radar cross-section (√ΔRCS) rather than the conventional delta radar cross-section (ΔRCS), since the √ΔRCS is field-based and preserves the phase, whereas the ΔRCS is a scalar power quantity. As the inherent self-interference in monostatic setups limits the measurement system’s dynamic range, additional carrier cancellation hardware is often employed. However, since this work targets a setup using only off-the-shelf components and measurement equipment, we follow a different approach: a bistatic arrangement combined with spherical wave expansion (SWE) theory to relax the dynamic range requirements and to allow for full-sphere measurements by pattern stitching. Since the bistatic configuration introduces an uncalibrated signal path, a two-step calibration procedure is developed to recover a monostatic √ΔRCS from bistatic measurements. Three different bistatic configurations are realized and compared, resulting in a measurement system capable of accurate full-sphere characterization with high dynamic range. The complete chain is validated against a full-wave simulation, in the field and in the modal domain, and applied to a label on a plastic crate with and without metal cans. As expected, results show a pronounced direction-dependence of the tag’s phase response, which is an important input for phase-based localization algorithms.