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This paper presents a phase-based method for obtaining the distance of an RFID tag with unknown properties. The tag's response is measured by the reader at several discrete frequencies at the threshold power of the tag. The dispersive properties of the modulated reflection coefficient of the tag are estimated from the measured power sensitivity and their effect is compensated in the distance estimation. The method is derived using a simple theoretical model for the tag and the performance of the method is experimentally verified at 860 MHz.