Compact directional coupler based on substrate integrated waveguide

A compact directional coupler based on substrate integrated waveguide is proposed. This coupler consists of two narrow apertures on the common broadside wall of two adjacent SIWs and four microstrip stepwise impedance convertors. Attractive features including compact size and planar form make this coupler structure easily integrated in antenna beam-forming networks. A coupler operating at Ka-band is designed. The coupling is about 7 dB at 31 GHz, the isolation is below 18 dB over the frequency range of 30.55-31.2 GHz, and the return loss is better than 10 dB from 30.3 to 31.4 GHz. Design considerations and results are discussed and presented.