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.

I. Introduction

Substrate integrated waveguide (SIW) filters are widely used over the past years because of their many advantages such as high , perfect isolation, planar structure, and ease of fabrication [1]. However, their use in compact wireless devices is hindered due to their large area consumption. Accordingly, miniaturization methods need to be applied to achieve more compact filters while maintaining the high performance characteristics. In [2], [3] quarter mode SIW (QMSIW) filters are introduced as a method of miniaturization. By cutting the full-mode SIW on its two fictitious magnetic walls, a QMSIW cavity resonator can be obtained which operates at a very similar mode. Hence, size reduction of approximately 75% is achieved compared to conventional SIW cavity resonators.