Various modes of the equiangular spiral antenna

The 2-, 4- and 6-arm planar spiral antennas were excited by various feeding arrangements, and the pattern, phase variation and polarization of their fields were measured. It was found that the modes predicted by the solution of the infinitely-many-arms spiral antenna could be excited by spiral antennas that had only a small number of arms. In particular, we succeeded in exciting the "unpreferred" mode of the spiral antenna. Measurements also showed that in addition to the main mode of the spiral antenna, the field also contains higher order modes, which actually dominate the main mode in certain regions.