Design of broadband microstrip-to-CPW transition

A broadband microstrip-to-CPW transition is presented. In the proposed structure, the upper microstrip ring is vertically coupled to the central conductor of the lower circular CPW via electromagnetic coupling. To obtain a wide bandwidth of operation, a short-ended stub is introduced. The introduced short-ended stub combined with the open-ended microstrip ring constructs a quarter-wavelength resonator. A sample transition has been designed and measured. Experimental results indicate that over 129% bandwidth with better than 10 dB return loss and below 1.7 dB insertion loss can be obtained.