Planar Feeding Structure for Dielectric Image Line Based on Forward-Wave Directional Coupler

This paper proposes a forward-wave directional coupler based on the coupled mode theory. This coupler couples power between a microstrip line and a dielectric image line (DIL), and achieves complete energy transfer from the microstrip line to the DIL. It can be used as a fully planar feeding structure for DIL. The major advantage of this structure, compared to other feeding layouts, is that it generates very little radiation, and can maintain low radiation when feeding a DIL with relatively low thickness. The design presented in this paper operates at a center frequency of 31 GHz. It has a small loss of 0.8 dB at best, and a 3d-B coupling bandwidth of 21.8% (27.8-34.6 GHz).