Wideband Probe-Type Waveguide-To-Microstrip Transition for 28 GHz Applications

A design of a broadband probe-type waveguide-to-microstrip transition operating in the 26-30.5 GHz band is presented. The transition is based on a combination of a simple low-profile waveguide adapter with the WR28 flange and a PCB containing a printed probe, a microstrip line and a short Substrate Integrated Waveguide (SIW) section in-between that is used for impedance matching improvement. The transition is formed only on the top dielectric layer of a multilayer hybrid PCB which consists of a high frequency material Rogers RO4350B with the thickness of 0.168 mm and four layers of traditional low-frequency and low-cost FR4 material. Performance of the designed transition was optimized using full-wave electromagnetic simulations and experimentally verified on two back-to-back test structures each containing two designed transitions separated by a microstrip line with the length of 20 and 30 mm. Measurement results show that the designed waveguide-to-microstrip transition has the bandwidth of 26-30.5 GHz for the -15 dB level of the reflection coefficient with the insertion loss less than 0.8 dB.