Study of X-Band Broadband Branch-Line Coupler Utilizing Two-Stage Coupled-Transmission Lines

In this paper, we describe branch-line couplers (BLCs) with broadband characteristics over relative bandwidths of 50% in X-band. The proposed couplers consist of a conventional BLC and external matching networks composed of two-stage open-circuited coupled transmission lines and an impedance step between the coupled lines. By utilizing the equivalent admittance approach to the proposed coupler, the wideband characteristics of the desired relative bandwidth can be obtained in the X-band. In addition, it is confirmed that the relative bandwidth is 50.5% in the electromagnetic simulation. Furthermore, prototype experiments are also conducted, and the validity of this design annroach is verified from these results.