Hybrid non‐uniform‐ Q lossy filters with substrate integrated waveguide and microstrip resonators

Three lossy filters with flat passband are proposed with substrate integrated waveguide (SIW) and microstrip resonators, designed with the non-uniform- Q method. By properly selecting the lower- Q -to-higher- Q ratio, the 0.2 dB bandwidth of the first filter is obtained to be 215 MHz (17% improvement), which is equivalent to a uniform unloaded Q -factor of 820 at the cost of additional in-band insertion loss (IL) of 0.5 dB, when compared with the full-SIW filters. Further, two techniques are utilised to adjust the Q -factor ratio. One method is implemented by mounting a resistor on each microstrip resonator to reduce its unloaded Q -factor. Then, the 0.2 dB passband of 243 MHz is achieved for the second filter, equivalent to a uniform unloaded Q -factor of 1420. The other method is realised by adjusting the Q -factor due to radiation loss of microstrip resonators. By changing the input/output feeding structures, the 0.2 dB bandwidth of the third filter is enhanced by 32%, at the cost of minimum IL increased by 0.9 dB. The occupied area of the proposed planar hybrid filters is reduced by 15-21% and the passband flatness is much better than that of the uniform- Q filter counterparts, as demonstrated by the simulated and measured results.