Air-Filled Substrate Integrated Waveguide (AFSIW) Filter with Asymmetric Frequency Response

A filter with asymmetric frequency response based on air-filled substrate integrated waveguide (AFSIW) is introduced. The proposed filter is low-loss, compact, and allows the introduction of a transmission zero in the upper-side. The transmission zero is implemented using the multilayer feature of the AFSIW technological platform as the secondary path of the signal is located in the bottom substrate of the structure, while the primary path is located in the inner substrate. Furthermore, the robustness of the structure against printed circuit board (PCB) standard processes highlights the high interest of the structure. For demonstration purposes, a third-order AFSIW filter with an asymmetric frequency response operating at 21 GHz with a fractional bandwidth of 1.53% is fabricated and measured. The measured results are in good accordance with the simulation results in and out band results. The fabricated prototype exhibits measured insertion loss of 0.66 dB with an unloaded Q-factor of 1429, which is excellent for a planar structure.