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The existence of asymmetric resonant modes on standard log-periodic dipole antennas is established experimentally. These modes are characterized by sharply-resonant side radiation sometimes accompanied by reduction in front-lobe gain. The resonances occur at frequencies whose relationship is approximately log-periodic. In a single antenna the resonant modes are excited by any structural asymmetry, while in an -plane array each individual antenna excites the others asymmetrically. The resonances can be eliminated by the addition of lossy material to appropriate parts of the antenna. Qualitatively the phenomenon is explained in terms of transmission-line resonances along the two-wire lines formed by adjacent dipoles. Automated swept-frequency far-field measurement techniques were employed throughout, and their worth is clearly established for broadband antenna research and development.