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The design of a frequency agile microstrip patch antenna is described that is readily interfaced with differential RF transceivers. By integrating three pairs of varactor diodes with the patch antenna, and tuning them in unison, frequency tuning ratios approaching 2 are possible with the design. This is made possible by the intrinsically broadband nature of the differential feeding scheme used. An intuitive equivalent circuit for predicting the port characteristics of the antenna is presented. Moreover, the circuit is shown to be highly accurate in predicting losses produced by the varactor diodes and the consequent radiation efficiency of the antenna. To the best of the authors' knowledge, this is the first time a detailed analysis of the effect of varactor diode losses has been undertaken in frequency agile antennas using an equivalent circuit. The equivalent circuit model is subsequently validated using full-wave simulations and experimental measurements of an antenna operating in the 2-4 GHz range.