The bandwidth of helical antennas

The propagation along a finite helix is examined by using a propagation constant which was determined by Sensiper for the infinite helix. By utilizing the Hansen and Woodyard condition, the bandwidth variation with the dimensions of the helix is obtained. It is shown that for a pitch angle
\psi
equal to
13\deg
, the normally accepted bandwidth of
0.77 < C_{\lambda} < 1.25
, where
C_{\lambda}
is the circumference in terms of free-space wavelengths, is applicable only to antennas up to about two wavelengths long. A curve showing the bandwidth as a function of length has been computed, and the shape of this curve has been confirmed experimentally. Good patterns and axial ratio with sidelobe levels no greater than -10 db were obtained for helices as long as 10 wavelengths. Good patterns and axial ratio may also be obtained for
\psi
as small as
2\deg
provided that the helix is sufficiently long to establish the surface wave.