A wideband balanced filtering coupled-line coupler with improved directivity based on coupled microstrip-slotline

In this article, a wideband balanced filtering coupled-line coupler (BFCLC) with improved directivity is designed. By using coupled microstrip-slotline, the proposed BFCLC realizes tight coupling while maintaining good isolation. A lattice equivalent circuit is introduced for the coupled microstrip-slotline. Good differential-mode transmission and high common-mode suppression are achieved through the application of microstrip/slotline transition structures. Additional stubs are taken to compensate for the odd- and even-mode phase velocities of the coupled microstrip-slotline. Transmission zeros can also be adjusted to improve selectivity. Based on the proposed BFCLC, a coupler with a smaller size using solely the slotline is designed. Two proposed couplers are manufactured to validate the theoretical design, and a good agreement between the simulation and measurement is presented.