Low-loss and broadband coaxial line to air-filled substrate integrated waveguide transition

This Letter introduces a low-loss and broadband coaxial line to air-filled substrate integrated waveguide (AFSIW) transition for the first time. The fabrication of this transition involves three substrate layers. The inner conductor of the coaxial line is connected to the metal layer of the lower substrate and its outer conductor is connected to the metal layer of the upper substrate. This transition uses a taper-shaped configuration at the end of the line to achieve the better impedance matching and wider matching bandwidth. In the previously reported AFSIW components, several transitions are used to transform the transverse electromagnetic mode of the coaxial line to the TE 10 mode of the AFSIW structure. By using the proposed transition, these extra transitions are eliminated, which leads to the size and loss reduction of the structure. For validation purposes, a prototype of the back-to-back coaxial line to AFSIW transition is fabricated and measured for the entire C-band. The simulated and measured results are in good agreement. The proposed transition achieves the measured insertion loss of 0.55 +/- 0.5 dB (0.28 +/- 0.25 dB for the transition) and return loss of better than -11 dB from 4 to 8 GHz.