Quasi-Reflectionless BPF and BSF for ISM Band with Enhanced Out-of-Band Characteristics

This paper presents quasi-reflectionless filters with bandpass (BPF) and bandstop (BSF) responses designed for the ISM band using a coupled line approach. The technique employs a bi-directional diplexer topology with a resistive element to at-tenuate reflected RF signals, achieving reflectionless performance. The reflectionless filters, designed with a center frequency of 2.4 GHz and fabricated on a low-cost FR4 substrate, demonstrate ex-ceptional out-of-band performance. Simulations were performed using Ansys HFSS and AWR Microwave Office, and measurements were conducted with a vector network analyzer (VNA). The BPF features a 3-dB bandwidth of 60 %, while the BSF achieves a bandwidth of 78%. Both filters exhibit simulated and measured return losses exceeding 10 dB from DC to 5 GHz. The proposed reflectionless filter designs provide a straightforward, efficient, and cost-effective solution for ISM band applications, offering minimal signal reflections and enhanced out-of-band performance.