Abstract: Broadband coaxial to rect-waveguide transitions are key components in modern high-quality antenna-feed systems. To meet the high-frequency and wideband requirement of modern electronic systems, a novel configuration of broadband coax to waveguide transition is described with double ridged wavewguide, untouched transition from the coaxial probe to ridged waveguide and tapered transition from the ridged waveguide to rect waveguide. Ridges are loaded in parallel on the same broad wall of waveguide. Different influences about the structural parameters of waveguide, ridges and SMA connector are discussed via contrast curves. In the measured bandwidth 6~20GHz, less than 1dB of the insertion loss and lower than -18dB of the return loss can be obtained. This transition can be empolyed in the feed and measurement applications of broadband antenna-feed system.