Parasitic effects and polarization diversity of Archimedean spiral antenna

For the purpose of controlling the polarization of an Archimedean spiral antenna (SA), a parasitic spiral element is added on the same plane of a two-wire SA. The radiation characteristics have been studied numerically and experimentally. First, the current distributions on both the active and parasitic elements are calculated from Hallen-type integral equations in the frequency range of 3 to 6 GHz. Comparing the current distributions with those of a two-wire SA and a four-wire four-terminal fed SA, the mechanism of polarization control is studied. The present antenna operates as a linearly polarized element at lower frequencies and a circular polarization element at higher frequencies. The input impedance does not change appreciably in frequency and hence the broadband nature of the conventional two-wire SA is retained. Circular polarizations in the opposite sense can be obtained over a wide range of angle due to change in excitation phase when four arms are excited.