Analysis and Design of Wideband 90° Microstrip Hybrid Coupler

A wideband 90° microstrip hybrid coupler with a bandwidth of 84.3% is proposed. The hybrid coupler consists of a two-section coupled-line power divider and a 90° phase shifter. The two-section coupled-line is of natural broadband characteristics, and not only performs the role of impedance matching, but also makes the total circuit compact. The phase shifter employs a pair of open- and short-circuit stubs inserted into the reference transmission line to compensate the phase variation in proportion to the operating frequency for the conventional transmission line. Thus, the phase shifter possesses wideband performance on impedance matching as well as phase shifting. To prove the design methodology, a wideband 90° hybrid coupler operating at 3 GHz is designed and fabricated. In the case of -15 dB reflection coefficient for all ports of the coupler, the output maximum amplitude unbalance is less than 1 dB, the phase variation is less than 5 degree, and the port isolation is better than 18 dB.