Spreading Antennas Around

[SirUlli]

Story by by Seth Shostak, SETI Institute

As SETI researchers are quick and keen to point out, the Allen Telescope Array, currently under construction about 200 miles northeast of San Francisco, is the first professional radio telescope designed from the get-go to speedily search for extraterrestrial signals. When completed, it will comprise 350 antennas, spread over roughly 150 acres of lava-riven real estate.

That?s a lot of antennas and a good chunk of property, too. But exactly where on that acreage will the antennas be erected? In other words, how will the array be arrayed? It will come as no surprise when I tell you that there?s a siting plan: a topographic map covered with 350 dots indicating where each antenna will be located. But whenever I unroll the plan under the nose of an interested party, a thought balloon with a question mark inflates above their head. ?That looks so random?? they blurt out.

And indeed it does. The plan resembles the holes on a dartboard ? and one used by a less-than-expert player at that. Folks are dismayed because they assume that the antennas should be on some sort of regular grid.
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Full Story

http://www.seti.org/site/apps/nl/content2.asp?c=ktJ2J9MMIsE&b=194993&ct=1125747

Sir Ulli

 

[Ken g6]

This is why I've sometimes said the OGR project is the real SETI@Home. For a linear telescope array (like one might want to use in space for finding planets), an OGR is (I believe) the optimal spacing setup.

 

[Assimilator1]

Wow! thats one big array!:Q

You've lost me ken

 

[JonB]

OGR (one of my favorite old projects) was pretty cool. Optimal Golomb Rulers was a mathematical method of spacing things along a line or even in a grid such that the sum and differences of the distance between the chosen points were all different. You may say, "why does that matter?" Go ahead, say it!

In antennas and radio frequncy electronics in general, when you use amplifiers, they are susceptible to frequency interpolation (sometimes called Beat Frequencies). For a field of antennas (antenni?) such as this, when the received signals are combined and processed there will be false frequencies in the signal. These will be introduced because radio waves only travel at the speed of light and would therefore be reaching the antennas at different times. At high radio frequencies, this is significant and causes phase differences and frequency shifts that make it difficult to determine signal source location. When antennas are arranged in an Optimal Golomb pattern, the phase differences and frequencey differences would all be different across the array and would be very weak. In a rectangular (checkerboard) grid layout, the phase differences would all be the same and would add together to appear almost as strong as the actual signal.

 

[Ken g6]

OGR. It's still going, you know.

P.S. Thanks, SirUlli!

 

[Assimilator1]

Thanks Jon

I know a little about radio (via semi amateaur radio useage) ,but I didn't know what you meant by frequency interpolation.....is that what you've just explained? lol