An X‐band centrosymmetric microstrip bandpass filter with reflectionless range over 3.9–18.6 GHz and stopband up to 40 GHz

Abstract

AbstractIn this letter, the design of a centrosymmetric absorptive bandpass filter (ABPF) with ultra‐wide reflectionless band at both input and output port and wide stopband is reported. It is based on a balanced‐circuit structure, consisting of a pair of 3‐dB couplers and two channels of conventional reflective bandpass filters (BPFs). The bandwidth of the 3‐dB couplers is maximized, which covers from C‐band up to beyond Ku‐band, to extend the reflectionless band of the ABPF. It is realized by introducing a cross‐fingered defected ground structure (DGS) into the classical coupled‐line coupler to strongly enhance the coupling effect while maintaining a compact size. To suppress the harmonics outside the passband, two C‐shaped DGSs are employed at the input and output ends of the coupled‐line‐based BPF which introduces extra transmission zeros in the stopband. A prototype of the proposed ABPF is fabricated and tested. Measured input/output return loss are better than 10 dB over 3.94–18.63 GHz, which is commonly viewed as the reflectionless band. Moreover, the upper stopband is extended up to 40 GHz with suppression higher than 23 dB. The presented ABPF features advantages in convenient integration, dual‐port ultra‐wide reflectionless band and broad stopband.