Miniaturization design for planar hybrid ring couplers

Miniaturization of a microstrip rat race coupler is implemented by three approaches. First, a new rat race hybrid design is devised for circuit miniaturization based on the periodic stepped-impedance ring structure. The circuit has a normalized area of 21.5%. In the second approach, miniaturization is carried out by incorporating a broadside-coupled structure and steppedimpedance sections into the design. The former is used to replace the 3λ/4-section and the latter is to substitute each λ/4-section in the regular 6λ/4 circuits. As a result, one of the circuits occupies only 18% of the area of a traditional rat race. Finally, a general synthesis is performed to design reduced-length rat race rings. Incorporating the stepped-impedance configuration, the normalized circuit area is 13.12% of a 6λ/4-ring. All the designs are validated by comparing measured four-port parameters with the simulated results.