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 -1^st resonance for more miniaturization. The 2-pole filter has a center frequency of ~690 MHz with an area of ~0.05λ₀ × 0.09λ₀. The measured insertion loss is 2.1 dB and return loss is better than 30 dB. By employing the -1^st 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.