Balanced multiple-layer dual-mode substrate integrated waveguide filters with controllable finite-transmission zeros

This paper proposes three balanced multiple-layer dual-mode substrate integrated waveguide (SIW) bandpass filters. The first balanced filter has two substrate layers, and two pairs of balanced feeding ports are located on the upper substrate layer. The lower and upper dual-mode SIW cavities are coupled by two crossed slots in the middle metal layer. The second balanced filter has two substrate layers, and two pairs of balanced feeding ports locate parallelly on the upper and lower substrate layers, respectively. The third balanced filter has three substrate layers, and two pairs of balanced feeding ports locate vertically on the upper and lower substrate layers, respectively. The proposed three balanced filters can produce three, two and one finite-transmission zeros (FTZs), respectively. Characteristics of the proposed multiple-layer SIW filters with controllable FTZs' locations are analysed in detail. For the demonstration, some design examples with the centre frequency of 7.5 GHz and bandwidth of 240 MHz are presented, and three design examples are fabricated to verify the performance of the proposed balanced filters. The measured results agree well with simulated ones.