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A high-efficiency 3-D harmonic repeater for narrow band, wireless sensing applications is described herein. This repeater consists of a diode doubler and conjugate-matched 3-D receive and transmit antennas. The antennas are designed on the surface of a cube in part to provide a convenient volume for packaging the sensor electronics. The device operates by receiving a 2.4-GHz signal and re-radiating a 4.8-GHz signal and has been optimized for an RF input power level between - 30 and -20 dBm with zero dc power. The design enables the return signal to be readily modulated with very low ( <; 0.1 V) voltages. The diagonal of the entire repeater measures 0.24 λ at the fundamental frequency of 2.4 GHz, and its measured conversion gain is -13.3 dB with a 2.5% 3-dB conversion gain bandwidth at -30-dBm input power. Given this performance, the expected communication range using a 2-W source is >; 50 m in a free space environment. To the best of the authors' knowledge, the presented harmonic repeater has the best overall performance among previously published designs, in terms of conversion gain, communication range, and occupied electrical volume. A new approach for remote calibration of passive sensors using orthogonal polarization of the interrogation signal is also proposed in this paper.