Axial-mode wire helix with linearly tapered ends
Image of the Axial-mode wire helix with linearly tapered ends.

The Axial-mode wire helix antenna with linearly tapered ends consists of a linearly tapered bottom section, a uniform middle section and a linearly tapered top section, mounted on a circular 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. [Angelakos and Kajfez]

Comparing the axial ratio and impedance response of this antenna to an ordinary axial-mode helix highlights the advantage of tapering the ends. One disadvantage is that the tapered ends reduces the gain when compared to an ordinary axial-mode helix.

Polarisation-specific gain pattern at the centre frequency
Axial ratio of antenna with tapered-ends compared to ordinary axial-mode helix
Input impedance of antenna with tapered-ends compared to ordinary axial-mode helix

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.