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
The beauty of mounting a parabolic antenna for SETI use is that you just can't go wrong. Since we are interested in monitoring the sky for artificial signals from beyond, the antenna merely need be pointed up -- there are stars (with potentially habitable planets) to be found in all directions. So mounting an antenna for SETI use is considerably simpler than, for example, using the same antenna for satellite TV, where it must be precisely aimed at the satellite's location in the sky.
Because there are no wrong directions for SETI, many SETI antennas are simply set on the ground, "bird-bath" style, looking straight up. But a disciplined sky survey, such as The SETI League's Project Argus effort, requires coordinated sky coverage, and that in turn necessitates a limited steering ability for at least some of the antennas in the network.
Where steering of the antennas is desired, we need to consider two degrees of freedom: azimuth (the compass heading to which the antenna points), and elevation (the angle which the antenna's beam makes with respect to the horizon). In terms of celestial coordinates, azimuth of a radio telescope (along with a station's latitude and longitude, and the date and time) determines the Right Ascention (RA) of its target, while elevation (again, along with lat/lon, time and date) determines Declination (Dec). Conversion between terrestrial and celestial coordinates is handled by this spreadsheet.
Since we live on a rotating planet, the Earth herself makes a most cost effective RA rotor, as long as we are willing to be patient and let the proper portion of the sky eventually rotate into view. But since (thankfully!) the Earth doesn't rotate north-to-south, the only way to acheive Dec control is to physically rotate the antenna along a north-south line. This can be accomplished by aligning a satellite TV antenna's position rotor as a vertical (elevation) rotor, as described in this article.
When antenna rotors are desired, the dish positioners commonly used for satellite TV will require some source of power. The operating voltage for these positioners can often be supplied by a surplus C-band satellite TV receiver, which may also have a digital readout of dish position. Alternatively, a separate DC power supply can be used. The required voltage is typically 24 to 36 VDC, and these rotors draw anywhere from one to four amperes. Polarity determines the direction of rotation, so switching should be provided to move the dish both up and down (or alternatively, both right and left).
Further information about mounting SETI antennas may be found in the Antennas and Feedhorns chapter of The SETI League Technical Manual.
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