Narrowband Substrate Integrated Waveguide Bandpass Filter With High Selectivity

In this letter, a highly selective narrowband bandpass filter (BPF) is presented in substrate integrated waveguide (SIW) technology. Higher order TE301 resonators are used to utilize their high unloaded Q-factors. A midline feeding scheme together with a bypass coupling provides three transmission zeros (TZs). Two of the zeros can be placed at band edges for high selectivity. A three-pole BPF with 3-dB fractional bandwidth of 1.1% at
f
₀
= 8.25 GHz is fabricated. It provides at least 35-dB attenuation over 2 GHz above and below the passband starting from
f
₀
± 0.15 GHz.