Mixed-Mode Substrate Integrated Waveguide Bandpass Filter with Controllable Transmission Zeros
A novel methodology for controllable coupling coefficients, especially changing the sign, is presented in this paper. Transforming the coupling coefficient to the diagonal of the coupling matrix, the response can be controlled by resonant frequencies shifting. As an example, a fourth-order bandpass filter implemented by the fundamental mode TE101, and the first degenerate modes, TE102 and TE201, is investigated theoretically. A pair of transmission zeros can be switched from the imaginary axis to the real axis through resonant frequencies shifting to generate two distinctive responses, quasi-elliptic and linear phase. For experimental validation, the mixed modes bandpass filter using substrate integrated waveguide (SIW) structure with two transmission zeroes at both sides has been manufactured and measured. It is designed at 9.5 GHz with 2% fractional bandwidth. The measured results are well agreed with simulated results.