Compact CPW rat-race and branch-line hybrids utilizing slow-wave structure

This paper presents structures of artificial coplanar waveguide (CPW) with very slow phase velocity and their applications to a design of compact rat-race and branch-line hybrids. The slowwave structure is constructed by periodically loading a CPW with simultaneous series inductance and shunt capacitance, and a meaningful reduction of the effective wavelength for a Bloch wave is achieved along with an adequate Bloch impedance. Next, a rat-race and a branch-line hybrid are designed using the artificial CPW’s, and consequently considerable sizereduction of about 1/2 to 1/4 is obtained.