Bandwidth improvement of rat-race couplers having left-handed transmission-line sections
A novel broadband rat-race coupler has been investigated. The coupler utilizes an artificial left-handed transmission line section for broadband phase response realization. Moreover, a narrowband model of left-handed section has been shown to prove the couplers equivalent circuit at the center frequency. To broaden the operational bandwidth multisection quarter-wave transformers have been proposed. The exemplary rat-race coupler with two-section impedance transformers has been designed and manufactured.  2013 Wiley Periodicals, Inc. Int J RF and Microwave CAE 24:341347, 2014.
broadband rat-race couplers left-handed transmission lines impedance transformers