Lossy substrate integrated waveguide filter with flat passband

A resistively coupled lossy filter is proposed using the substrate integrated waveguide (SIW) resonators, where nonresonating nodes are avoided for simple realization. Based on the sensitivity analysis of the resistive coupling coefficient, the parameters of coupling structure and mounted resistors are determined. In the measured results, the 0.2-dB passband is increased from 198 MHz of the case without resistive couplings to 256 MHz with resistive couplings. The flat passband and high selectivity are achieved at the cost of an additional insertion loss (IL) of 1.1 dB. Since SIW cavities can provide higher unloaded Q-factor than microstrip resonators, a smaller in-band IL deterioration will be required with the lossy SIW filter than microstrip counterparts when a good passband flatness is desired, especially for narrow-band applications. Excellent agreement is observed between the simulated and measured results.