SETI League Technical Manual -- Block DiagramCopyright © 1999 by H. Paul Shuch, Ph.D.
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @ setileague.org
When radio waves strike a dish antenna, the parabolic shape of its reflector directs all the energy to a single point out in front of the dish, called its focus. The purpose of the feedhorn, which is mounted at the focus, facing the reflector, is to scoop up all this energy, and apply it to the LNA and receiver for processing.
The most common feedhorn for amateur SETI use is a metal pipe, closed off at the end farthest from the dish, forming a shorted cylindrical waveguide. The horn contains a small metallic probe, connected to the center pin of a coaxial connector, to collect the energy and apply it to the input connector of the LNA. The horn may be surrounded by a metal ring, used to improve the efficiency of energy collected from the surface of the dish, or to block interference from entering the feed from beyond the periphery of the dish, as described in this article.
The chief drawback of the cylindrical waveguide feedhorn is that its large physical size actually blocks a part of the dish surface from view of its incoming signals, effectively reducing the size (and hence the gain) of the parabolic antenna. This blockage loss is most severe for small dishes, becoming almost negligible at the popular 1.4 to 1.7 GHz SETI frequencies when the dish diameter exceeds about four meters.
An alternative to the waveguide feedhorn is the helical feed, consisting of about three turns of heavy wire in a corkscrew shape, with a circumference of one wavelength at the operating frequency, and a spacing between turns of a quarter wavelength. A helix feed doesn't block the aperture of the dish to the extent that a waveguide horn does, but is more prone to interference from signals off to the side of the antenna. Both helix and waveguide feedhorn designs have been used successfully by SETI League members.
Additional information on various SETI antenna feeds, along with vendor links, may be found in the Antennas and Feedhorns chapter of The SETI League Technical Manual.
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