A multipole-field study for the multiarm log-spiral antennas

The electromagnetic field radiated by an arbitrary source is represented as the multipule expansion of the left-hand circularly polarized and of the right-hand circularly polarized modal fields. In particular, the multiarm log-spiral antennas have been investigated using such an approach. With approximated current distributions, the multipole coefficients and the circularly polarized field patterns for a few typical log spirals are computed and compared with the measured results. Generally, only a few dominant modes are sufficient to describe the major radiating characteristics of the log spirals; therefore, a better insight as to the polarization and the angular dependence can be gained through the modal-field point of view.