A novel compact multi-layer MMIC CPW branchline coupler using thin-film microstrip stub loading at 44 GHz

A very compact 90/spl deg/ CPW branchline coupler at 44 GHz for Ka/K-band, advanced SATCOM terminal systems operating in the 20/44 GHz frequency bands is reported. This coplanar coupler structure using two metallization layers separated by a thin dielectric sheet on GaAs was investigated. This arrangement provides additional degrees of freedom compared to standard coplanar line structures resulting in extremely compact couplers suitable for monolithic integration. The proposed method takes advantage of the coexistence of the CPW lines and shunt thin film microstrip stubs. The coupler does not use lumped inductors or capacitors and, subsequently, has the advantages of excellent design accuracy even at high frequencies. In comparison with conventional hybrid couplers, this coupler offers lower insertion loss, higher compactness (a significant 65% reduction of the circuit area) and additional degrees of freedom by the inclusion of the shunt thin film microstrip stubs. These stubs are the sole reason for the major reduction in size.