Size-reduction of coupled-microstrip 3 dB forward couplers by loading with periodic shunt capacitive stubs

Downsizing of microstrip coupled-line 3 dB forward (codirectional) couplers are proposed. An enlargement of the difference of the effective phase velocities for the coupled-modes is accomplished by loading a coupled micrstrip with periodic shunt capacitive stubs, and consequently the length of the coupled line section is considerably reduced to 18% of a traditional coupled line section. Furthermore, a flat coupling, or a broadband performance, is achieved by use of asymmetrically loaded coupled line section. Finally, the design results are confirmed both experimentally and numerically.