A Low-Loss Via-Free Ridge Air-Filled Substrate Integrated Waveguide on Printed Circuits Board for mmWave Application

A novel via-free ridge air-filled substrate integrated waveguide (VF-RAFSIW) structure is first proposed on printed circuit board in millimeter wave (mmWave) band. With copper paste (CP) and electroplating, the proposed VF-RAFSIW is entirely enclosed by copper; it can be built up with low-cost FR4 substrate in a typical printed circuit board (PCB) process. The ridge design makes the VF-RAFSIW more compact and more stable in electric performance when compared to the conventional RAFSIW. The cutoff frequency and propagation constant can be determined with the Ritz–Galerkin method. The back-to-back transition structures from microstrip to VF-RAFSIW are fabricated and measured for several lengths. By avoiding using the vias, the width of VF-RAFSIW is reduced by 46% compared to that of conventional AFSIW. The measured attenuation constant of VF-RAFSIW is 0.27 dB/cm at 60 GHz. The excellent performance of propagation loss reveals that the VF-RAFSIW is a good candidate for transmission structure in mmWave and sub-THz bands. Finally, a phase shifter designed in the VF-RAFSIW technology is demonstrated to show the feasibility of this platform.