Abstract: A side-launch transition for efficiently joining a coaxial transmission line or coaxial connector to a microstrip transmission line for operation at millimeter wave frequencies. The microstrip transmission line comprises a conductive microstrip pattern on one side of a dielectric substrate and a ground plane on the other. The conductive microstrip pattern includes a rectangular terminal pad area of a predetermined length designed to be approximately equal to the wavelength of the signal being transmitted. This pad area is integrally joined to the microstrip line. A circular iris or aperture, is formed through the ground plane and is in a centered alignment with the terminal pad on the opposite side of the substrate. A small hole extends through the center of the iris and penetrates the substrate as well as the conductive pad area.