Inline Ka-band Transitional combline / evanescent-mode filter in conventional RF substrate using grounded vias

The Ka-band is considered a viable band for future 5G backhaul links. Planar filters in this frequency range have low aspect ratios for substrates thicker than 10 mil, since the required 50 Ω line width becomes comparable to a guided wavelength. This paper proposes the use of grounded through-hole plated vias as transitional combline / evanescent-mode resonators. The principle is demonstrated with a 4^th order, 27.5-29.5 GHz filter in a conventional 32 mil thickness RF substrate. The simulated filter features below 2 dB insertion loss across the band and occupies 3.8 × 1 mm board space.