Evanescent‐mode bandpass filter using D‐shaped slotlines for spurious harmonic suppression

A novel miniaturized evanescent‐mode substrate integrated waveguide (SIW) bandpass filter with D‐shaped slotlines for spurious harmonic suppression is proposed in this letter. Closed‐form equations for the proposed evanescent‐mode SIW cavity are summarized and two D‐shaped slotlines are etched together as a pair by the face‐to‐face style to obtain the designed lowpass feature, which could be utilized to improve the out‐of‐band spurious harmonic rejection level. To validate the feasibility of this method, a miniaturized evanescent‐mode SIW filter with D‐shaped slotlines is designed and measured. The measured results show that this filter operates at 1.05 GHz with 2.4 dB insertion loss and 3.8% bandwidth. The out‐of‐band spurious rejection is lower than 16 dB up to 11 times of the higher cutoff frequency of the passband. All test results are in good agreement with the simulation results.