A new method of exciting a two-wire spiral antenna from two off-center sources is proposed in order to obtain the polarization diversity. It is found both numerically and experimentally that as the locations of two sources are shifted from the center of the spiral towards the arm ends, the radiation wave changes from a right-hand to a left-hand circularly polarized wave, by way of a stage in which a linearly polarized wave is produced. On the basis of the obtained results, the frequency characteristics of a spiral antenna with two fixed sources are investigated. It is found that the spiral antenna radiates a right-hand circularly polarized wave at a frequency of f" and a left-hand circularly polarized wave at a frequency of 2f".