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A novel technique for radio frequency identification devices (RFID) based on remote measurement of complex impedance at microwave frequencies is presented. The low cost read-only tags do not need semiconductor elements and can be implemented by printing a conducting pattern on a low cost dielectric substrate. Multiple tags are simultaneously illuminated by a chirped microwave signal with a bandwidth of few hundred MHz to few GHz. The composite backscatter from all tags is mixed with the transmitted chirp signal and filtered to generate an intermediate frequency (IF). The IF is further processed with a narrow detection bandwidth to select a particular tag. Next, the phase-frequency profile of the reflection coefficient (complex impedance) of the selected tag is recovered with the help of a reference channel. Phase-frequency signatures of individual tags determine the tag identity. The technique allows operation with low transmit power, provides range information and mitigates against multi-path propagation and interference.