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Experimental results of the effects of dielectrics on the radiation pattern of electromagnetic horns are presented. The experiments were conducted with triangular and modified triangular shapes of dielectric materials. Because of the ease of fabrication, styrofoam was chosen for the dielectric. Furthermore, the low dielectric constant ( ) of this material reduces the signal reflections at the air-dielectric interface. The dielectric was placed inside a 7-foot long -plane sectoral horn operating at -band frequencies. Near- and far-field measurements showed that the dielectric, in spite of its low dielectric constant, changed the phase and the power distribution across the aperture of the horn. By judicious choice of the shape and material of the dielectric, an antenna beam with sidelobes below -40 dB can be obtained.