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Why 1000s of acres antenna needed?

  1. Jul 29, 2009 #1
    Hi,
    I read the following on seti.org:

    "Our earliest TV broadcasts have reached several thousand nearby stars, although any alien viewers would have to build a very large antenna (thousands of acres in size) to detect them."

    I guess that signals have got very weak....( does it mean amplitude is very small that it was when it was transmitted?)....but what difference is the size of antenna going to make?
     
  2. jcsd
  3. Jul 29, 2009 #2

    Vanadium 50

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    A weak signal means a small amount of energy per unit of surface area. If you want to collect more energy to detect the signal, you therefore need more surface area.
     
  4. Jul 29, 2009 #3
    Wow I never thought about it
    That means aliens can know quite much about us
    How long can they reach to be clear enough anyway?
     
    Last edited: Jul 29, 2009
  5. Jul 29, 2009 #4

    Born2bwire

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    Vanadium's got the long and short of it. Most antennas are not very directional by themselves, except perhaps a parabolic reflector antenna (dish). However, even an antenna that radiates in all directions equally can be used to create a very narrow beam of reception and transmission. This is done by using an antenna array, muliple antennas placed at predetermined spots being fed by signals that can be shifted in phase and changed in amplitude. If you have a very weak signal, you want to use an antenna system that has a very high gain (highly directional) so that you will not only receive just the signal of interest but at a larger amount of received power than a single antenna can do alone.

    Take a look at the Very Large Array antenna. I'm sure you've seen pictures of it, to get an idea of an enormous antenna array that is used to grab extraterrestrial signals. Keep in mind that all of the very large dish antennas in the VLA all work together to receive (and send if that was their fancy) the same signal.
     
  6. Jul 29, 2009 #5
    The thermal noise in a RF/microwave receiver preamp operating at room themperature is about
    kT (Boltzmann's constant times temperature) = 4.14 x 10-21 Watts (-174 dBm) for a 1 Hz bandwidth. It is possible to cool solid state preamps to a few kelvin and reduce this. But if the received signal is say 10-25 watts per square meter and has a bandwidth of say 10,000 Hz, what you need is acres of antennas.
     
  7. Jul 30, 2009 #6
    Thank you all for the replies.


    I found this about VLA:
    The VLA is an interferometer, which means that the data from each antenna can be combined electronically so that the array effectively functions as one giant antenna.

    Then, as Vanadium said, it would effectively be of a large surface area and collect more energy.
     
  8. Jul 30, 2009 #7

    berkeman

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    Keep in mind that the VLA is used for collecting much more energetic EM information than parasitic TV transmissions from a single planet outside our solar system. The VLA is used to image much larger things than a TV broadcast station on Alpha-Centauri-3...
     
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