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Beam width

Discussion in 'Electronic Design' started by Adam, Feb 10, 2007.

  1. Adam

    Adam Guest

    Hello everybody,

    I have heard that in the radar and radio communication there is a law
    of radiation. The larger aperture of the aerial (or the more number of
    the aerial)
    The narrower beam you'll have. As I have heard it is true for NDE
    ultrasonic transducer which means if you use an array of transducers
    you'll have a narrower beam of ultrasound rather than using just one
    of those transducers. I am not able to understand the reason?
    It seems there is a paradox there!
    Suppose an ultrasonic transducer has 60 degrees of beam width, if you
    use an array of 500 of these transducers then you'll get 5 degrees of
    beam width or even less while you have a very much large array instead
    of just one transducer!!!

    Is there anyone to direct me why the beam separation is much less for
    an array of transducers rather than just one transducer?

    Many thanks for any help
     
  2. Ian Stirling

    Ian Stirling Guest

    1.22* wavelength / diameter of antenna gives the emitting angle (in
    radians)

    The antenna is comprised of _all_ the emitting components.
    Multiple little antennas spread over an area can have a
    significantly narrower beam, approaching the beamwidth of an antenna
    covering entirely that area.

    Google 'radio telescope interferometry'
     
  3. Tim Wescott

    Tim Wescott Guest

    No paradox. The best attainable beam width is related to the ratio of
    the antenna size to the wavelength. It's a fundamental limit of any
    optical system, or any other system that's based on waves.

    Google "diffraction" or "diffraction limit". There should be some
    information there.

    Then do the math.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  4. Tim Wescott

    Tim Wescott Guest

    Actually, that figure gives you the best emitting angle that you can
    expect -- you can always mess it up by using the wrong antenna design.

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at http://www.wescottdesign.com/actfes/actfes.html
     
  5. Ian Stirling

    Ian Stirling Guest

    Yes, it'll never be that good.
     
  6. default

    default Guest

    http://en.wikipedia.org/wiki/Phased_array
     
  7. Fred Bloggs

    Fred Bloggs Guest

  8. default

    default Guest

    I don't know what the original poster was suggesting. No doubt with
    500 emitters all hit with the same signal you'd have plenty of
    interferences, but the beam width wouldn't be smaller unless the
    signals were phased.

    Anyhow he googled in and probably won't be back.
     
  9. Adam

    Adam Guest

    default äæÔÊå ÇÓÊ:
    It would be for an arry of ultrasonic transducers withought phasing
    them..

    Lan Stirling,
    Yes I am aware about that formula but can not understand the reason!?
     
  10. Adam

    Adam Guest

    Well, perhaps it is better to ask my question with another statement,
    Why the opening of an ultrasonic transducer is important for
    directivity of the wave?
    Yes I know this formula:
    (sinx= 1.2 (wavelengh/opening

    But why that is true?
    Suppose the wavelength of the wave is smaller than the opening of the
    source anyway it would be spread after propagation similar to a
    wavelength which is larger than the opening!
    How know what the mystery which I don't know is? What is the real
    story....!?
     
  11. The formula applies for all transducers in phase, of course.

    Rene
     
  12. http://en.wikipedia.org/wiki/Angular_resolution#Explanation
     
  13. Tim Williams

    Tim Williams Guest

    Imagine the wave front passing the opening. No matter what angle the
    incident waves are travelling, the waves passing through the opening will
    always be angled in the direction the hole is pointing. This works for
    relatively small holes. For a very large hole, it might as well not exist
    and the incident waves will propagate on through in the same direction they
    started. These are two very different behaviors and the point where they
    start to change is when the hole is within a factor of perhaps 5 of the
    wavelength.

    Tim
     
  14. Adam

    Adam Guest

    Tim Williams äæÔÊå ÇÓÊ:
    But as i said the a wave without any angle of propagation will be
    spread out in the air too, so?

    Does anyone know the above formula for "N" of transducers?
     
  15. joseph2k

    joseph2k Guest

    really good. The best comparable ideas were phased array radar, and
    synthetic aperature radar.
     
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