Design and Analysis of Novel Bandpass Filters with Adjustable Center Frequency and Bandwidth

This paper presents a novel design of microstrip bandpass filters combining stepped-impedance resonators (SIR). Initially, a stepped- impedance resonator is considered as the basic structure, and then the basic design is progressively modified and improved. These enhancements include adding resonators to the basic structure, modifying their dimensions, fundamentally changing their arrangement and connections, and creating air gaps between resonators. These modifications result in significant changes in the frequency response characteristics, shifting the center frequency of the basic model to the X-band. Subsequent steps focus on improving filter parameters such as bandwidth, insertion loss, and return loss, ultimately leading to a dual-band bandpass filter. The single-band filter proposed in this paper has a center frequency of 9 GHz, a bandwidth of 1.83 GHz, an insertion loss of 1.7 dB, and a return loss of 54.6 dB. The dual-band bandpass filter features center frequencies of 7.4 GHz and 10.7 GHz, with bandwidths of 110 MHz and 530 MHz, insertion losses of 1.4 dB and 2.3 dB, and return losses of 22.4 dB and 31.1 dB, respectively. The designed filters are characterized by compact size, high integration, low insertion and return losses, and wide bandwidth. A good agreement is observed between the measured results of the fabricated sample and the simulation results. Keywords: Bandpass filter, microstrip, SIR resonator, air gap, insertion loss, return loss