Rat-race based microstrip coupler with differential port to realize monostatic RF systems

In this work a rat-race based microstrip coupler, usable to realize monostatic RF systems, is presented. The coupler combines the two functions of separating the transmit (TX) and receive signal together with the conversion from a differential to a single-ended TX signal since two ports of the proposed five port coupler can be used as a differential port. The two functions are realized in a single microstrip structure, leading to decreased losses and a space-saving layout. Simulations and measurements at 79GHz confirm the function of the coupler.