Ask Dr. SETI: Condition of Surplus Dishes

The SETI League, Inc., a membership-supported, non-profit {501(c)(3)}, educational and scientific organization Searching for Extra-Terrestrial Intelligence

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Ask Dr. SETI ®

Chapter 6: Technology

Condition of Surplus Dishes

Dear Dr. SETI:
I read your questions and answers about dish accuracy. My question is maybe more general. I am new to SETI and want to build my own search station. I started by looking for a used TV satellite dish. I found more than one available at different prices. So I want to know what kind of things should I look for and consider in a dish?
I figure some factors are: size, good condition, mesh vs. fiberglass, mounting, attached equipment and feed horn, weight (I want to put it on the garage roof), and accuracy. My considerations involve value for price.
How do you recommend I look for a dish? Since these are used and have to be taken apart (or are already apart), how do I determine condition?

Lee (SETI League member)

The Doctor Responds:
Gain correlates closely to size, so first determine the largest dish which you can accommodate in your available space, and look for one of that size. As a rule of thumb, doubling the diameter increases gain by a factor of four, and doubling distance increases free-space path loss also by a factor of four. So all else being equal, a dish twice the diameter will hear exactly twice as far. But because the angular coverage is inversely proportional to gain, at any given time that "twice as big" dish will cover only a quarter as much of the 4 pi steradians of space. Thus for achieving full sky coverage, the smaller dishes have their place; they sacrifice distance for acceptance angle.

As for dish condition, the main consideration here is surface accuracy. If the surface of the dish is dimpled, dented, or distorted, 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.

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!