Broadband Rat-Race Couplers With Coupled-Line Section and Impedance Transformers

Design of broadband rat-race couplers with the use of a shorted coupled-line section and impedance transformers has been comprehensively investigated. A broadband rat-race coupler utilizing a shorted coupled-line section and two shorted quarter-wave long transmission lines has been modified allowing for achieving ideal isolations between two pairs of respective ports. Also it has been shown that the bandwidth of such couplers can be increased when multisection impedance transformers are applied, and that the Darlington synthesis procedure can be used for calculation of transmission-line impedances. A tabular data for rat-race couplers with up to four-section impedance transformers is given. The theoretical analysis has been verified by measurements of the manufactured broadband rat-race coupler operating within 0.8 to 2.8 GHz frequency range.