This helix antenna consists of a linearly tapered bottom section, a uniform middle section and a linearly tapered top section, mounted on a conical ground plane.
The end-tapers serve to match the feed and the termination of the uniform helix to the feed line and free-space, respectively, while the central uniform helix operates as an ordinary axial-mode helical antenna. These matching techniques improve the axial ratio and impedance behavior of the uniform helix by reducing the generation of unwanted modes. The conical ground plane further reduce sidelobe levels, while also allowing for impedance control. [Angelakos and Kajfez]
A comparison of the gain versus angle response highlights the advantage of using a conical groundplane to suppress sidelobes. The impedance versus frequency response illustrates that the input resistance of the antenna using the conical groundplane is reduced and essentially constant over the band. The conical groundplane also reduces the negative reactance of the antenna....
D. J. Angelakos and D. Kajfez, “Modifications on the axial-mode helical antenna”, Proceedings of the IEEE, vol. 55, no. 4, pp. 558–559, Apr. 1967.