A coaxial-to-microstrip vertical transition includes a dielectric substrate having formed on a first surface thereof a primary microstrip conductor trace, and on a second surface a secondary microstrip conductor trace. A first conductive via extends through the dielectric substrate and electrically connects the primary conductor trace to the secondary conductor trace. A second conductive via is spaced from the first conductive via and extends through the dielectric substrate to electrically connect the secondary conductor trace to the coaxial center conductor. A bottom microstrip ground plane layer is defined on the second substrate surface. A conductive base plate structure has a cavity formed therein, the substrate positioned such that the base plate structure is in contact with the bottom ground plane layer, and the secondary conductor trace is positioned over the cavity. The substrate is positioned between a cover structure and the base plate structure, the cover structure disposed in spaced relation with respect to the first surface of the substrate. A coaxial transmission line structure includes an outer shield and a coaxial center conductor structure disposed within the outer conductor and transverse to the substrate, the center conductor passed through an opening in the cover structure to contact the second via. A conductive plate structure is positioned between the plane of the cover structure and the substrate, providing shielding surrounding the center conductor between the cover and the substrate.