Asymmetrical Impedance Inverter for Quasi-Optical Bandpass Filters With Transmission Lines of Fixed Length

Quasi-optical (QO) filter design is commonly based on the direct coupled bandpass filter topology with distributed resonator elements. Conventionally, prototypes with symmetrical impedance inverters are employed during synthesis, and as a result, this leads to resonators with custom lengths across the filter. This can be impractical from a QO filter implementation perspective, since spacers of arbitrary thickness may not be readily available. In this paper, we propose a novel impedance inverter equivalent network, which allows the development of prototypes where transmission lines across the filter structure to have predefined fixed lengths. Based on this technique, we developed a fourth-order QO filter prototype that composed of 2-D periodic arrays with subwavelength features. Its performance is verified with free-space measurement, and the measured results show good agreement with both full-wave and circuit simulation. Thus, it is shown that the proposed impedance inverter circuit can simplify the implementation of QO filters of higher order.