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This paper presents a new design to improve the transmission characteristics of coaxial-to-microstrip transitions. A metallic ring is added to a conventional coaxial structure to serve as a buffer between a coaxial line and a microstrip line. The introduction of the metallic ring reduces the insertion loss of the transition caused by the sudden changes of the electromagnetic field distributions from one transmission line to another. This improvement is more prominent at higher frequencies; hence, the 1-dB transmission passband of the transition is increased significantly. A practical design example is implemented and measured. The increase of the 1-dB passband is more than 60%. The new design is also proven to be suitable for transitions between various coaxial cables/connectors and microstrip lines on different substrates or another commonly used planar transmission line, the coplanar waveguides. The robust nature of this design makes its performance not susceptible to the fabrication and assembly errors of the transitions. These characteristics qualify the proposed design as an excellent candidate for coaxial-to-microstrip transitions at higher frequencies.