The logarithmic spiral in a single-aperture multimode antenna system

The
n
-arm logarithmic spiral antenna having
n
distinct terminals may be considered as an (
n-1
)-port antenna. It may be operated simultaneously in (
n-1
)independent balanced modes. By varying the relative excitation of these modes it is possible to produce a variety of patterns that have applications to direction finding, tracking, and homing over a broad range of frequencies. It is particularly easy to obtain useful results in this manner because of some unique properties of the logarithmic spiral. The basic relations between mode excitation and the related fields of these multiarm antennas are reviewed together with some properties that are characteristic of the log-spiral geometry. Multimode single-aperture systems, such as those using a relatively simple hybrid matrix to synthesize multiple beams tilted from the axis of the antenna, are discussed. These tilted beams can be conically scanned about the axis of the antenna by a simple phase shift within the excitation matrix.