Swept-frequency radiation pattern anomalies on helical antennas

Swept-frequency radiation patterns for several helical antennas indicate that both the lower and upper edges of the passband are marked by the occurrence of a series of anomalies responses which are characterized by very narrowband fluctuations in the swept-frequency gain and axial ratio plots. The upper bandedge gain anomalies coincide with the reflection coefficient and axial ratio variations reported in the literature. The occurrence of both the upper and lower bandedge anomalies is shown to be related to two mode stop regions in the helix dispersion diagram. Several suggested tapering schemes have been tested and it is shown that the lower bandedge anomalies, unlike those which occur at the upper gandedge, cannot be controlled by front-edge tapering. A modification of the feedpoint and backplane cavity which suppresses normal-mode generation is required to affect the lower bandedge anomalies.