Transitional combline/evanescent-mode microwave filters

Traditional combline-filter theory based on TEM-mode coupling results in bandwidths which are too large-the increase being a function of the ground-plane spacing to wavelength ratio b/λ. As the b/λ ratio increases from 0.05 to 0.35, the bandwidth ratio (BWR), defined as actual bandwidth/TEM bandwidth, increases from unity to over 2:1. This bandwidth increase has now been confirmed by calculation of the coupling coefficient between combline resonators using mode matching. Accompanying the increased bandwidth is a considerable increase in the unloaded Q in accordance with the higher Q of the evanescent waveguide modes. A similar study carried out in the case of interdigital filters shows that these effects are much less significant