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An Archiedean planar spiral antenna is numerically analyzed in the presence of a conducting plane reflector. The analysis shows that the spiral antenna backed by the plane reflector has two distinct regions in the current distribution, which explain the radiation of a circularly polarized wave for the outer circumference C ranging over shout 1.3 X < C < 1.5 A and C > 2.9 X, where A is a free-space wavelength. Further consideration is given to a truncated spiral antenna whose outer circumference is on the order of 1.4 A. The truncated spiral antenna maintains a decaying current distribution and radiates a circularly polarized wave over a k1.2 frequency bandwidth. It is also demonstrated that a power gain on the order of 8.5 dB is realized over the same frequency range.

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