Compact Balanced Substrate Integrated Waveguide Filter With Low Insertion Loss

A balanced substrate integrated waveguide (SIW) filter is proposed by etching transverse slots on the top surface. The SIW structures between each two adjacent slots are resonators operating at TE $_{20\delta }$ mode for differential-mode (DM) operation and at TE $_{10\delta }$ and TE $_{30\delta }$ modes for common-mode (CM) operation. The proposed TE $_{20\delta }$ -mode resonator is much shorter than the traditional TE201-mode resonator, which causes the whole design to be compact and low loss. Furthermore, the strong electric coupling formed by slots make it have wider bandwidth when compared with the state-of-the-arts. A prototype is designed at 3.5 GHz with the size of $1.2\lambda _{\mathrm {d}} \times 0.83\lambda _{\mathrm {d}}$ ( $\lambda _{\mathrm {d}}$ is the wavelength in dielectric at the center frequency), the minimum insertion loss of 0.91 dB, the 3-dB fractional bandwidth (FBW) of 16%, and the FBW for 20 dB CM suppression of 79%.