ABSTRACTThis article presents a miniaturized rat‐race coupler with harmonic suppression. The proposed coupler only takes 15.1% of the area of the corresponding conventional rat‐race coupler. By replacing the traditional transmission line with the proposed stub‐loaded transmission line equivalently, the coupler performances can be retained and size can be reduced significantly at the same time. In addition, due to the transmission zero introduced by the stubs, the harmonic suppression is also improved. To verify the effectiveness of the substitution, the design equations are derived for analysis. Finally, the proposed coupler is fabricated for experimental verification, and a good match is found between the simulation and measurement results. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:190–193, 2015