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See The Horn Antenna of Holmdel Rotate

Radio Test Engineer Peter Guerlain explains how the Horn Antenna in Holmdel works, and how it was used to confirm the existence of cosmic energy left over from the "Big Bang." On July 10, Steve Wolkovitz captured the rare sight of its movement.

The first communications between the radio satellite Telstar 1 consisted of a 65-foot dish antenna in Andover, Maine and the historic Horn Antenna on Crawford Hill in Holmdel. Last Thursday, scientists celebrated the 50th anniversary of Telstar next to the Horn Antenna.  


As is so often the case scientific discoveries often occur "by accident", or, to put it more accurately, as a by-product of an endeavor to do something else. 

And, as with most any scientific measurement or experiment, once one has set up the equipment, one starts out by checking, or calibrating the equipment to make sure that it is working correctly so that one can rely on the results produced by that equipment.

For example, if the equipment is a thermometer, one might first check it by sticking it into ice water then boiling water to verify that it reads 32 then 212 degrees F and if it does, one can be reasonably expect it to provide accurate indications between and around those temperatures.
 
The was a part of a system to receive extremely faint radio signals.  Several interesting techniques were used to make the system so sensitive.  One was to make it extremely directional, which provides the dual advantage of not only being able to "hear" better in a certain direction, but also to hear less from other directions, thus improving the ratio between what you are trying to hear and other things that might interfere. 

It's like trying to listen to a conversation in one part of a room full of people while one is stuck in another part of the room; if one were to ignore the social implications and cup one's hand behind one's ear and turn one's head just so, one could hear better in a desired direction while blocking out some of the sound from the nearer conversations.

Another technique was to cool the radio receiver--a key capability that applies to any radio receiver is its sensitivity, or ability to receive the weakest signal.  That limit is a function of noise generated within the receiver itself, that is, since it is a physical thing it is made of molecules and all molecules have parts that move- indeed temperature is simply a measure of how fast those parts are moving.

Those movements produce noise, or energy at radio frequencies that interferes with the signals one wants to hear, thus limiting the sensitivity of the receiver.  So the scientists built a special radio receiver that could be cooled to extremely low temperatures with liquid nitrogen and helium, to slow down the molecular activity and make the receiver as sensitive as possible.

A third technique was simply to make the antenna quite big, the bigger it is the more power it can capture, just like putting something bigger than your hand near your ear to collect and reflect more sound into your ear.  Indeed, the horn antenna is much like an ear, it has a sensor, which is analogous to one's eardrum and that sensor is relatively small, the horn is simply a big reflector that collects and focuses radio signals upon the sensor.

At this point, the scientists had a receiving system that was so directional and sensitive that they needed to confirm that the limiting factor was the temperature of what they aimed it at, indeed if they aimed it at the ground they would receive radio noise from the molecules that make up the ground or if they aimed it at the moon or stars they would pick up radio noise from their molecules.

 It's like using an ordinary traffic cone as a hearing aid—anyone can try holding one to their ear and it is remarkable how one can hear distant conversations, traffic and the like.  Then try finding a direction to aim the traffic cone where you don't hear anything at all--it's difficult.  On a calm night, that direction just might happen to be straight up into the sky, (so long as no airplanes are flying overhead) because if one aims it at the ground, though you will not hear molecules with your ear, the ground might still reflect sounds coming from other directions into the cone. 

So the scientists calibrating the receiving system did essentially the same thing as in the examples above--after cooling the receiver, they aimed it at things of known temperatures such as the ground then the darkest part of the sky they could, to confirm that the noise would go to expected levels then decrease to an expected level, thus confirming the accuracy and sensitivity.  But try as they might, they could not get the noise coming out of the receiver to decrease to the level they expected.

 After checking and re-checking everything, they finally realized, and were the first to confirm, that the noise was of a level that cosmologists had calculated would have to exist in space if the universe was created in accordance with the Big Bang theory the noise, though faint, was real, not something wrong with the receiving system, and they had made something capable of receiving it!

Ray July 15, 2012 at 04:05 PM
Great article. It explains such a complicated thing in such a simple clear way. I really enjoyed reading it.
Deannie July 15, 2012 at 06:25 PM
....Thank you Peter Guerlain for the very well-written article. The analogies to situations/experiences we all know enables the lay person to under this complex scientific event.
William Myers July 16, 2012 at 06:59 PM
Arno Penzias received the Nobel Prize for Physics in 1979 for this discovery, along with Robert Wilson; Stephen Hawking said the Nobel should have gone to Wilkinson at Princeton for his efforts in finding the Cosmic Microwave Background Radiation. He was wrong about the Higgs Boson and he was wrong about that too: Penzias didn't just stumble across this, he'd lectured on the CMBR at Princeton in the 50's. The history behind this discovery is amazing.

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