Double resonance of broadband microstrip-to-waveguide transition in millimeter-wave band

Broadband microstrip-to-waveguide transitions are developed in the millimeter-wave band. Two types of novel transitions are developed first. One is a quite broadband transition with an air back-short waveguide in a metal block. In order to avoid the use of an additional part of the metal block, we developed a transition in the multi-layer substrate. Double resonance is a key feature for the broadband operation. It is due to the double stubs not only of the back-short waveguide but also of the parallel-plate waveguide in the substrate. Consequently, bandwidth of the transition is extended by optimization of the stub length in the substrate. Furthermore, equivalent performance is obtained even when via holes are replaced by choke structure. The performances of these transitions are summarized in this paper.