This device provides for a substantially orthogonal transition or connection point between a strip transmission line configuration (one conductor, or two conductors sandwiched around a dielectric support, positioned in air between two ground planes such as might be used in a microwave antenna feed network) and a coaxial line section. The particular construction of the "side-launch transition" suppresses spurious parallel plate modes, and, in the case of two conductors surrounding a dielectric, trapped modes which occur between the two conductors. A U-shaped upper "dam" and corresponding lower "dam," electrically interconnected at several points, surround the termination of the stripline conductor(s) and provide suppression of parallel plate moding at the transition. In the case of two conductors surrounding a dielectric, electrical interconnection at the terminal point of the stripline conductors suppresses the trapped modes before they can be launched. The system has been found to provide extremely low voltage standing wave ratios over a wide range of microwave frequencies.