Broadband Transition from Microstrip Line to Waveguide Using a Radial Probe and Extended GND Planes for Millimeter-Wave Applications

A broadband microstrip line-to-waveguide (MSL-to-WG) transition is developed for E-band applications. In order to achieve a sufficient and broadband coupling between the microstrip line (MSL) and waveguide (WG), a radial electric probe at the end of the MSL and extended ground (GND) planes on the dielectric substrate are proposed. Results are compared against a simple transition (S-Tr) with a straight electric probe. For the case of operational bandwidth (BW) for an input return loss (S11) below −20 dB, the proposed transitions using the radial probe and extended GND planes show the BW enhancement of 33.8% and 61.9%, respectively, compared to the S-Tr. The proposed and simple transitions were fabricated on a low-loss liquid crystal polymer (LCP) dielectric substrate. The measured bandwidth (BW) for S11 below −10 dB of the proposed transition is over 28GHz, which is satisfied at all test frequencies from 67 to 95GHz. Its measured insertion loss can be analyzed as −1.33 and −1.41 dB per transition at 70 and 80GHz, respectively, considering the loss contribution of the cable adapter and waveguide transition.