Center Fed Helix

Center fed helix at 3 GHz.

When the diameter of a center fed helix is around one third wave length, standing waves along its single element like those on

an over-moded halfwave dipole will radiate circularly polarized radiation in both directions along the helix axis. The antenna calculator displays the current along the element and the Poynting vector of radiation in the three coordinate planes. It displays the polarization of the axial radiation, and the directivity of the structure.

A four turn helix is of length 8 cm and diameter 3.7 cm. The scale of length indicated by yellow + is 10 cm. Radiation emitted in the XY and YZ planes is shown in blue. Target dA is placed along the helix axis ( zenith= 0 ) at range 1 m ( 10 wavelengths ). The peak current along the element is 1.418 mA, providing 1 mW total radiated power.

The instantaneous current at point 30 indicated by the asterisk along the radiator is about 1.24 mA mostly in the in the y direction.

The circularly polarized fields across dA and along dr are shown. The current is presented in the spherical coordinate system as mostly in the azimuthal (phi) direction. The electric, magnetic, and Poynting fields at the center of dA are shown in agreement graphically by dA, dr, and numerically in the table at lower right. The Poynting vector at the target is .11 mW/m^2 in the z direction, away from the helix.

The standing wave along the element is shown The position of element 30 along the element is shown as a local current minimum around 1.24 mA at the center feed point. The current along the element is animated by the points in Source. The current nodes and maxima in visible in Source match those indicated graphically in element_helix

The circular polarization of the radiation at the target is shown in dA.

The operating wavelength is increased by 5 percent to eliminate the local current minimum at the feed point. The circular polarization and other parameters are changed as shown.

The wavelength is reduced by 3 percent to find the frequency of nearly perfect circular polarization.