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We have studied the effects of nearby objects on the read range of several types of RFID tags, and the impedance, pattern, and radiative efficiency of antennas that closely emulate the tag structures, using measurements and simulations. We find that the main reason for the decrease in read range at perpendicular incidence in close proximity to metals or dielectrics (such as water) is the decrease in electric field near the surface required to meet the boundary conditions. Detuning of the antenna plays an important role only when the tag is very close to the object. All tags tested show similar qualitative behavior, but significant quantitative distinctions in performance are observed for differing antenna designs.