E-plane Stepped -Impedance Bandpass Filter with Wide Stopband

This paper presents E-plane stepped-impedance bandpass filter with wide stopband. In comparison with the well-known E-plane filters on homogeneous resonators, the stopband of proposed structure is much wider and has higher attenuation. Significant improvement of these characteristics was achieved by a slight change in the topology of the filter resonators - the implementation of each resonator was realized using fin-lines of significantly different slot widths. These changes in topology dramatically change the frequency response of the resonators. In resonators with a significant predominance of the length of the low-impedance fin-line section, an increase in the bandwidth between the main and high order resonances can exceed 50% compared to homogeneous resonator. This is accompanied by a significant increase in the attenuation level in this frequency band and the appearance on the attenuation pole, the frequency of which can vary over a wide range by changing the impedance of the low-impedance section. The filter synthesis procedure is based on the use of a modified technique with K-inverters. The size of the elements required to implement the inverter specifications were calculated using a FDTD simulation. The two-resonator filter with a central frequency of 17.25 GHz, with a bandwidth of 300 MHz was calculated and manufactured; it showed losses in the passband of the order of 1 dB, losses of higher than 30 dB up to 29.5 GHz in the stopband, and a maximum attenuation of 45 dB in the stopband. In comparison, the calculated values for a two-resonator filter using homogeneous resonators with the same specifications produces the same stopband losses only up to 21 GHz, a maximum loss of 32 dB, and moreover, the proposed filter has a reduction in size of 25%.