An antenna apparatus comprises a substrate with a microstrip-to-waveguide transition comprising a microstrip feedline extending between a first terminal point and a second terminal point at a first metal layer and comprising a microstrip element and a probe element. The microstrip element includes a connection segment extending from the first terminal point to a second point, a taper segment extending from the second point to a third point, and a continuous-width segment extending from the third point to a fourth point. The probe element extends from the fourth point to the second terminal point and has a width which is narrower than the continuous-width segment. The substrate further includes a waveguide opening comprising a region surrounding the probe element and includes a plurality of metal vias disposed at the perimeter of the waveguide opening and which extend from the first metal layer to the second metal layer.