A way to obtain quasi-anti-phase excitation without a balun circuit is proposed for a two-wire spiral antenna, where an inner spiral-wire end is fed from a coaxial line and the other inner spiral-wire end is in contact with a ground plane. The radiation characteristics are found to be comparable to those of a conventional spiral antenna with anti-phase excitation. The frequency bandwidth for a 3-dB axial-ratio criterion is 18%, with a gain of approximately 8dB.
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