Abstract: Coaxial-to-microstrip transitions may include a microstrip line and coaxial-line assembly. The microstrip line includes a first dielectric having an aperture, a conductive strip disposed on one primary face of the first dielectric, and a ground plane disposed on the opposite primary face of the first dielectric. The coaxial-line assembly includes an outer conductor and an inner conductor. In some examples, the ground plane extends between the outer conductor and the inner conductor on a first side of the coaxial-line assembly proximate the conductive strip and an aperture cross section extends beyond the outer conductor on a second side of the coaxial-line assembly distal the conductive strip. In some examples, the ground plane has a non-circular aperture. In some examples, the outer conductor encloses an area that is less than an area of the aperture. In some examples, the enclosed area has a width that is less than a corresponding width of the first aperture.