Quasi-elliptic filter based on SIW combline resonators using a coplanar line cross-coupling

In this paper, a novel electric coupling for Substrate Integrated Waveguide (SIW) bandpass filter is proposed, designed and applied in combline SIW structure. The electric coupling is obtained by means of an open-ended coplanar transmission line which couples combline SIW resonators. The proposed coupling structure can introduce mixed electric and magnetic coupling thus helping to enhance the filter selectivity. To prove the concept, a quasi-elliptic cross-coupled 4-pole SIW filter is designed including transmission zeros (TZs) at both sides of the pass-band. The filter is centred at 5.75 GHz with absolute bandwidth of 114 MHz (2%). The TZs are set at 5.63 GHz and 5.87 GHz. The measured results confirm the performances of the proposed structure, showing the advantages of wider upper stop-band, higher compactness and easy integration with planar structures.