Impedance loading state determination for UHF Passive RFID Applications

In this paper, the scattering of a tag antenna situated in multipath environments and connected to different loading impedances is analyzed. The scattered field is contributed by three major components: the clutter field, the structure mode, and the antenna mode of the scattering antenna. We use a complex coordinate to graphically explain the relationship among the measured quantity and each contributing component. Experimental results show that the response difference between the two impedance states obtained by the coherent detection method is always greater than or equal to that obtained by the incoherent detection method.