A two-wire spiral antenna with unbalanced feed

A two-wire spiral antenna has been numerically analyzed in the presence of a conducting plane reflector. The analysis shows that, when the outer circumference is appropriately chosen, the antenna radiates a circularly polarized (CP) wave in the direction normal to the antenna plane. In practice, a balun circuit is required to achieve anti-phase excitation for the CP radiation in the normal direction. This paper proposes a novel unbalanced feed system, which achieves quasi-anti-phase excitation without a balun circuit. The antenna is fed from a single coaxial line. The radiation characteristics are numerically investigated and compared with those for a conventional spiral antenna with a balanced feed system.