Abstract: A novel structure in a coaxial to waveguide transmission line transition includes a waveguide section having longitudinally extending ridges on two opposed walls; a metal impedance transition member extending from one wall of the waveguide having a geometry within the waveguide of three right cylinders of dissimilar size stacked upon one another to form a steplike arrangement and a central passage therethrough; a dielectric cylindrical member extends into said waveguide from an opening in a waveguide wall opposed to that of the transition member and is in abutting relationship with the end of said impedance transition member; an elongate rodlike conductor extends axially through said dielectric member and said impedance transition; further each of the longitudinally extending waveguide ridges includes an outwardly flared portion of increasing width, one of which abuts the periphery of the lower step of said transition member and the other of which is spaced a predetermined distance from said dielectric member