Compact Rat-Race Coupler Using Shunt-Stub-Based Artificial Transmission Lines

In this letter, a compact rat-race coupler is developed using shunt-stub-based artificial transmission lines (SSB ATLs). A conventional quarter-wavelength microstrip can be equivalent to an SSB ATL realized by two series high-impedance microstrips with the high-impedance branch-type shunt stubs. The circuit configuration of the SSB ATL is beneficial to miniaturize the circuit size using the folding technique. By integrating six SSB ATLs, a compact rat-race coupler can be implemented with a 3.9% circuit size of the conventional one. The measured results of the developed rat-race coupler are in a good agreement with the simulated results by full-wave electromagnetic simulator.