Miniaturized Rat Race Coupler with Microstrip-to-CPW Broadside-Coupled Structure and Stepped-Impedance Sections

Miniaturization of microstrip rat race coupler is implemented by incorporating a broadside-coupled structure and stepped-impedance line sections into the design. The former is used to replace the 3lambda/4-section and the latter is to substitute the lambda/4-line in the conventional circuit. No lumped-element is required in the circuit. One of the experimental circuits occupies only 18.3% of the area of a traditional 6lambda/4-rat race. It is believed that it has the best size reduction in comparison with those in open literature. In addition to a wideband characteristic, total inband power loss is better than 3.2%. Measurement results are shown to validate the design and theoretical prediction.