A Novel Miniature Single-Layer Eighth-Mode SIW Filter With Improved Out-of-Band Rejection

An ultraminiature two-pole bandpass filter based on the eighth-mode substrate integrated waveguide is proposed in this letter. The combination of a coplanar waveguide and circular-ring miniaturized element as a new feeding method is used to improve the out-of-band rejection by introducing two transmission zeros. The measured insertion loss and return loss of the filter operated at 690 MHz are 1.25 dB and better than 20 dB, respectively. The out-of-band rejection is better than 30 dB up to 3 GHz. The equivalent circuit, simulation, and measured results are consistent with one another. The overall size of the filter is 35 mm × 35 mm, which is equivalent to 0.08λ0× 0.08λ0. A miniaturization factor of 98.8% is achieved.