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DirecTV dish-- F/? and dB gain?

Discussion in 'Beginner Electronics' started by DougC, May 7, 2008.

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  1. DougC

    DougC Guest

    I have been wanting to try some silly experiments using a digital TV
    dish to make a super-hearing type thing for a while, and finally got
    ahold of one from someone's trash.

    The one I found has the mounting pole and horn bracket, but the feed
    horn is missing. Even so just looking at photos of them online, it
    doesn't appear to me that the horn is at the geometric focal point.
    Also--even considering it's an offset dish--the dish appears rather
    shallow and has a round profile, not a parabola at all. Do RF waves
    behave differently than sound waves in this regard? (Would RF waves have
    a shorter F-length than simple geometry would indicate?)

    Also just for trivia purposes I am curious as to what the typical dB
    gain is for this sort of dish (when used for RF), and if that equates at
    all to sound waves...
    ~
     
  2. JeffM

    JeffM Guest

    A parabolic microphone (audio)?
    The sintered finish on many of those
    makes me wonder how good they will be for that
    --without a layer of smoothed epoxy.
    Your vision of how a parabola can look seems limited.
    http://www.google.com/images?q=conic-sections
    One can look like a comet or a gentle bow in a piece of sheet metal.
    Of course, to get a close-by focal point,
    it will have a pretty beachball-like curvature.

    Maybe you can get a better answer in a more apt group:
    http://groups.google.com/groups/sea...es+curvature+ingroup:audio+-FS+-sex&scoring=d
    This group is a rather broadly-oriented one to post that question.
    It also has very low traffic levels.
    It's not hard to find a more narrowly-focused group.
    http://groups.google.com/groups/search?q=dish+gain+-DougC
    ....and there's always Google Web Search.
    http://www.google.com/search?q=dish+gain&num=100
    That's an interesting question.
    If you find an answer elsewhere, be sure to post it here as well.
     
  3. Guest

    The feed horn is of course mounted exactly at the focal point, but the
    dish is not used perpendicular to the satellite. Combined with the
    offset mount for the feed horn, it all works out very fine - thank
    you.

    You wouldn't be able to discern the difference between a spherical
    section anntena or a true parabola without the abilty to measure
    within thousandths of an inch to the focal point of various diameters
    of the 18" dish. 10 foot C band are spherical-ish at best and no way
    to even measure the difference between perfection and what you get for
    $50 used. You are chasing much less than one tenth decible difference
    when the dish has 38 db gain at C band 3.7 Ghz - no point at all to
    such silly behavior. Toss the Parabola dreams where they belong.

    Exact db gain has to do with dish size and wavelength so no, it can't
    possibly relate to sound waves with 30 feet wavelengths compared to
    half inch RF wavelengths. I dont' have the formula at my fingertips
    but it's not a national secret, google for it yourself.

    There is a reason you don't see them much outside of a Monday night
    football game - they don't work as advertised. Better designs enclose
    or shut out sounds entering from the sides so the old parabola has to
    go right off. I think a 5 gallon plastic bucket would work just a
    good if not better just on extraneous sound rejection principles. You
    are dealing with a decided mismatch in wavelenths to focal point
    distances, a portable dish will never work out well for sound
    frequencies.

    Of more interest to me would be constructing an array of canceling
    microphones to extract the "aimed at" sound of desire - at least this
    one is plausible.
     
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