SETI League Technical Manual -- Block Diagram

Parabolic Reflectors

Copyright © 1999 by H. Paul Shuch, Ph.D.
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @ setileague.org

Though many other antenna types have been used successfully, by far the favored antenna for amateur SETI use is the parabolic reflector ("Dish"). The chief advantage of the parabolic reflector is that it operates over an extremely wide range of frequencies, limited at the low end by its diameter (which must be a respectable multiple of the longest wavelength being received, to provide reasonable gain), and at the high end by its surface accuracy (which must not deviate from the parabolic shape by more than a small fraction of the shortest wavelength being received, to maintain reasonable efficiency). Typical satellite TV dishes generally provide reasonable performance over the 1 to 10 GHz portion of the microwave window.

For reception in the 1.4 to 1.7 GHz region which is highly favored for much amateur SETI activity, the optimum dish size is on the order of three to five meters in diameter. In countries such as the US and Canada, where C-band satellite television distribution has been widely used for two decades, suitable dishes are abundantly available at low to no cost. In other parts of the world they are harder to come by, and enterprising SETI League members have acquired surplus commercial telecommunications dishes, or even built their own from scratch.

The size of the dish and the operating wavelength together determine antenna gain. As a first order approximation, the voltage gain (as a ratio) is equal to the circumference of the reflector, measured in wavelengths. Consider, for example, a 3 meter dish, which has a circumference of (3 * pi) = about 9.4 meters. At the 21 cm resonant wavelength of neutral hydrogen atoms (corresponding to the popular SETI frequency of 1420 MHz), the voltage gain of this antenna would approach (940/21) ~ 45. Since power ratio equals voltage ratio squared, the power gain of such an antenna would be about 2,000, which equates to +33 dBi of gain. (In fact, since the efficiency of amateur SETI antennas is generally on the order of 50%, the actual gain realized is more like +30 dBi.)

Dish size also determines beamwidth, which dictates the degree of aiming precision required when targeting specific stars. As an approximation, half-pwer beamwidth in radians equals wavelength divided by antenna diameter. Thus, for our prior example of a 3 meter dish operated at 21 cm, the beamwidth is on the order of (21/300) ~ 0.07 radians, or 70 milli-radians, which is about four degrees.

If you choose to obtain a surplus antenna, dish condition becomes an important factor. The main consideration here is surface accuracy. In order to perform up to expectations, a dish's surface cannot deviate from the parabolic by more than a tenth of a wavelenght. At 1420 MHz, that's about 2 cm of allowable surface error. If the surface of the dish is dimpled, dented, or distorted beyond 2 cm, avoid that dish! Look for something which approximates a smooth parabolic curve. If panels are missing or bent, performance is going to suffer.

Next, look at the mounting hardware. If it's rusted, you're going to have trouble getting the dish apart, and more trouble reassembling it. Weight is sometimes a consideration, as is wind loading. If these are concerns to you, a mesh dish may prove more realistic than a solid one.

Many of the accessories which come along with a satellite TV dish will be of limited use for SETI, and therefore you should not pay extra for them. C-band or Ku-band feedhorns and preamps are only useful if you're going to search in C-band or Ku-band (some of our members do; most prefer to scan the Water-Hole, in L-band.) TVRO receivers are great sources of microwave components, but unless other civilizations utilize exactly the same TV transmission standards we do, they're not particularly useful as SETI receivers. And a motorized mount which tracks the Clarke (Geosynchronous) orbital belt is not particularly useful for drift-scan, meridian transit mount radio telescopes, except if modified per the instructions in the Antenna Mounts section of this document.

In the final analysis, your budget will likely be your chief limitation, so go with what you can afford. Any dish at all will receive better than no dish at all!

Additional information on various SETI antenna options, along with vendor links, may be found in the Antennas and Feedhorns chapter of The SETI League Technical Manual.

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