Design of a Compact Harmonic Transponder Based on Quarter-Wavelength Impedance Transformers

In this paper, the optimised design of a fully passive harmonic transponder is presented. The proposed harmonic transponder is based on the use of quarter-wavelength impedance transformers. Two quarter-wavelength transformers are used to control the fundamental frequency and the harmonic in a traditional harmonic transponder. By adding one additional quarter-wavelength transmission line at the fundamental frequency and another at the harmonic (or one-eighth wavelength at the fundamental frequency), the input and output can share one port for communications. The device has very a small circuit size and high performance. Compared to a typical passive harmonic transponder with two independent antennas, the proposed harmonic transponder can reduce the limitation of the antenna design without increasing the circuit area and is beneficial for designers to improve the performance of the antenna. Measurement results show that a conversion loss of 15 dB can be achieved at an input power of −25 dBm and an operating frequency of 2.4 GHz. The circuit size is only 10 mm × 12 mm.