The results of experiments on the radiation characteristics of circular-waveguide-excited dielectric spheres with matched sphere-air boundaries are reported. The test data have shown that a waveguide-excited dielectric sphere, backed by a metallic hemisphere and with simulated quarter-wavelength matching at the dielectric–air boundary through surface perturbations, produces a directive pattern with a low side-lobe level, increased on-axis gain and reduced 3 dB beam-width, as compared with a simple wavegudie-excited dielectric sphere of the same diameter and an optimum horn with an aperture of identical cross-sectional area. Also, it presents a low voltage standing-wave ratio over the waveguide band.