The backfire-helical-fed parabolic reflector provides an effective means of obtaining a high-gain circularly polarized radiation pattern using a relatively simple and low-cost feed structure. In most instances, the helical feed may be held in place by a simple on-axis supporting post, circumventing the need for feed strut braces.
The design and placement of the backfire helix feed is critical. By using a small ground plane and selecting the pitch angle and number of turns of the helix correctly the required beamwidth of the backfire radiation to illuminate the dish can be achieved. The practical beamwidth variation range is relatively small when compared to a horn feed, limiting the range of F/D ratio parabolic reflectors that can reasonably be used for this kind of feed approach. ...
In practice, the coaxial cable used to feed the helix would be fed along an on-axis support post from the center of the reflector - for physically smaller antennas at higher frequencies, a semi-rigid feed coax may even provide the actual support for the feed element. The exact placement of the feed is typically adjusted experimentally during construction to ensure ideal placement at the focal point of the reflector.
Antenna Magus allows this antenna to be designed for gain or beamwidth at a specific frequency.The on-axis circular polarization performance of a typical backfire-helix-fed parabolic reflector antenna.