An ultra-miniature quarter-mode SIW bandpass filter operating at first negative order resonance
This paper presents an ultra-miniature bandpass filter based on quarter-mode substrate integrated waveguide (QMSIW) cavity with negative-order resonance. Ramp-shaped slots are used as interdigital capacitors (IDCs) to force the structure to operate in the negative order resonance mode. A patch with triangular slots in an additional middle metal layer along with disconnected vias is also used to increase the equivalent series capacitance of the IDC and shifting down the ??1st resonance for more miniaturization. The 2-pole filter has a center frequency of ∼690 MHz with an area of ∼0.05⋌ 0 × 0.09⋌ 0 . The measured insertion loss is 2.1 dB and return loss is better than 30 dB. By employing the −1st order resonance, measured out-of-band rejection of >30 dB up to ∼2.1 GHz is achieved. To the best of authors' knowledge, miniaturization factor of ∼91% compared to a conventional 2-pole SIW filter shows this is the most compact SIW 2-pole filter reported in the literature.