New circular-polarized frequency-independent antennas with conical beam or omnidirectional patterns

Abstract
A conical beam may be obtained from balanced equiangular spiral antennas by constructing an antenna with more than two spiral arms and symmetrically connecting these arms to provide a suppression of the radiated fields on the axis of the antenna. The angle of this conical beam can be controlled and, with proper choice of parameters, confined to the immediate vicinity of the azimuthal (\theta = 90\deg) plane. An antenna with four symmetrically spaced arms can provide a radiation pattern that is within 3 db of omnidirectional circularly polarized coverage in the azimuthal plane. The standing-wave ratio of this antenna referred to a 50-ohm coaxial cable is less than 2-to-1 over the pattern bandwidth. This four-arm version retains the wide frequency bandwidths of the basic conical log-spiral antenna, and it provides a coverage which heretofore has been difficult to obtain even with narrow-band antennas.

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