Design of re-entrant mode microstrip directional coupler for high directivity performance

In this paper, we implemented a novel re-entrant mode microstrip directional coupler for realizing the high directivity characteristic. Achievement of the high directivity with microstrip configuration was carried out by matching the even and odd mode effective phase velocities. By using 2-dimensional finite element (FE) calculations, the phase velocity for each mode and design parameters were extracted for given dimensions. Based on the extracted design parameter with phase matched condition, we designed and fabricated 30 dB directional coupler at 850 MHz. Experimental results show good with excellent isolation