Highly-Selective C-Band Rectangular SIW-Based Bandpass Absorptive Filter

A high-selectivity second-order bandpass absorptive filter is designed based on the rectangular substrate-integrated waveguide (SIW) technology. The proposed C-band filter is inspired by the complementary duplexer architecture and is designed at the central frequency of 5.8 GHz. The absorptive filter consists of a T-junction, and reflective bandpass and bandstop filters. The T-junction is exploited to divide the input path into two main and auxiliary channels to mimic the complementary duplexer architecture. The bandpass filter guides the signal within the passband spectrum of the filter to the output. Furthermore, the bandstop filter, with a carefully designed matching section, guides the wave reflected in the stopband of the bandpass filter to be dissipated in a matched load. The filter substrate is RO4003c with a dielectric constant of 3.55 and a thickness of 0.813 mm. The simulation and measurement results show that the 10-dB fractional absorption bandwidth is 14%, the passband insertion loss is 1.39 dB, and the return loss at 5.8 GHz is more than 30 dB. In addition, the 3-dB passband bandwidth is 3.6%, which denotes the superiority of the proposed absorptive filter in terms of selectivity compared to the similar reported results, while maintaining an acceptable insertion loss. The filter design is simple and systematic, and thus it is straightforward to increase its order and selectivity without increasing the number of resistors.