An “inline” quasi-elliptic substrate integrated waveguide filter with high stopband rejection

In this paper, a fourth-order single-layered substrate integrated waveguide (SIW) quasi-elliptic filter with inline resonator arrangement is proposed. Transmission zeros (TZs) are introduced by the cross-coupling between higher/lower order modes in the SIW cavities to realize the quasi-elliptic function response and high stopband rejection of the filter. Measured results show that the 3 dB bandwidth of the filter is from 19.65 to 20.15 GHz, or 2.5% fractional bandwidth. The passband return loss is better than 20 dB, and the minimum insertion loss is 2.9 dB. Good agreement between the simulated and measured results is obtained. By using the proposed filter structure, four TZs are introduced, and high selectivity and high stopband rejection are obtained.