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A three-layer dielectric rod (TLROD) antenna design optimized for stable realized gain, pattern, and phase center from at least 4 to 18 GHz is presented. Such antenna characteristics are useful for reflector feeds or for near-field range probes. Full-wave simulation results of the current optimized TLROD antenna exhibits stable gain around 10 dBi and 3-dB beamwidth of 60° in both E and H planes from 2 to 18 GHz. The phase center variation is within 0.25 inches from 4 to 18 GHz, and has a maximum variation of two inches occurring at 2 GHz. The major design challenge of TLROD antenna is to determine optimal thickness, dielectric constant, and tip shape of each dielectric layer for achieving desired VSWR, pattern, and phase center requirements. An autonomous design optimization procedure was developed based on genetic algorithm and novel fitness functions for simultaneously controlling the values and variations of reflection coefficient, phase center, and pattern characteristics over the desired wide frequency range.