small logo SETI League Technical Manual -- Antennas

How Many Dishes?

Copyright © 1995 by H. Paul Shuch
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @
Since its inception in 1960, SETI has been frustrated by limited availability of radiotelescope time to provide reasonable sky coverage. The typical research-grade radiotelescope can observe perhaps one millionth of the sky at any one time. Which means that, even should a given facility happen to be on the air at exactly the right frequency, at exactly the time an ETI signal reaches Earth, there's a 99.9999 percent chance we'll miss the call. To provide full sky coverage would require a million such radiotelescopes, which obviously the present leadership in Washington is not about to provide.

The SETI League, Inc. is trying to change all that. While we certainly can't imagine funding a million research-grade instruments, there is another way. The typical satellite TV dish covers perhaps one five-thousandth of the sky, so it stands to reason that a coordinated network of five thousand such small installations can achieve real-time full sky coverage. Our published goal of enlisting that many microwave experimenters is by no means arbitrary. That no such scheme has yet been accomplished should certainly not deter us!

The down side of the proposed sky survey is that antenna gain and beamwidth are inversely proportional. So all else being equal, an antenna which sees 200 times more sky than a giant radiotelescope is going to have to be 200 times less sensitive. This is why what I call the American Syndrome has prevailed in research-grade radiotelescope design: if a little is good, a lot is better.

From a practical standpoint, the reduced gain of amateur SETI antennas translates to reduced range. An extra-terrestrial signal which a major radiotelescope can detect at, say, 100,000 light years (the diameter of our galaxy) will still be visible to our small dishes, but at a range of only 500 LY. So amateur SETI will of necessity be concentrating on detecting those very strong (meaning probably nearby) yet highly intermittent signals of which the famed Ohio State Wow! signal is the best known example.

Yes, it would certainly be nice to have the added range with which larger antennas would afford us. On the other hand, what good does it do to have an antenna with all the gain in the world, if it's looking in the wrong direction when The Call comes in?

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