Absorptive k-band bandpass filter using a balanced recursive structure

This article presents a new K-band absorptive bandpass filter (ABPF) based on a microwave balanced recursive architecture. The proposed structure was configured using two passive microwave hybrid couplers, two conventional bandpass filters (BPFs), and a recursive path control module consisting of a phase shifter and an optionally variable gain amplifier. Using the proposed structure, stable return characteristics that were insensitive to the output load variation in the passband, a reduction in standing wave due to absorption in the stopband, and potentially high reliability could be achieved. Furthermore, since the same BPFs were reused, the electrical filtering order within the given physical BPF stages could be increased effectively. The proposed architecture was verified by comparing it with the performance of the conventional two-stage cascaded BPF. The measured results showed a 3 dB passband at 280 MHz with the center frequency at 19.9 GHz and improved roll-off characteristics. Furthermore, the stopband showed the reflectionless characteristic with the return loss being better than 7 dB.