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Am I getting multipath distortion?

Discussion in 'Electronic Design' started by _, May 1, 2008.

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  1. _

    _ Guest

    ....and if so what can I do about it?

    I have a cobbled-together setup that almost works. I coach swimming, and I
    have a small video gadget that links to a Tivo and a TV via one of those
    video transmitters that you can get from ebay. I think it is a 1.2 gHz
    50mw version.

    The kids swim and then watch, and having the Tivo means there's a
    continuous flow of action. The problem is that the picture quality is bad.
    I'm only 20 metres away and there is a direct line-of-sight from
    transmitter to reciever - ocassionally with a body or two in the way; but
    even without, there is considerable jumping and colour distortion and
    generally a bad picture. If I stand still at just about any distance I can
    find a place that gets a good picture, but moving - even turning - makes it
    change.

    The ceiling and the upper half of the inside walls of the pool are panelled
    with strips of aluminium sheathing - about 6 inches wide and 20 or more
    feet long. These are separated by about a 1-inch gap along the long edges,
    and mounted to a framework (which is mounted to concrete) with some sort of
    clip arrangement. This is a box about 220 feet long and 25 feet high.

    What's the best guess for improving the picture? More power? Different
    frequency? Antenna mounting (I could wear a beanie with it sticking up
    from the top - that'd be cool)?
     
  2. Wimpie

    Wimpie Guest

    Hello,

    When the multi path effect only affects received signal strength, you
    can find many places with good reception and many with bad reception.

    Best thing is to try it in an empty swimming hall. Your environment is
    stable in that case. Move slowly < 1"/s and see what happens. At 1.2
    GHz (WL about 10", 0.25m), the distance between bad reception and good
    reception can be as low as some inches.

    50mW is more then sufficient power for such an indoor application, so
    assuming reasonable receive and transmit antenna, increasing power
    will not help. You might switch to another channel or look to the
    picture with transmitter off to check for interference from a similar
    or other system sharing the same frequency.

    In case of severe multi path effects bad and good reception may
    alternate within a yard (1m). When in the good places the reception is
    noise free, signal will have sufficient strength.

    With severe multi path, where path via reflection can be many yards
    longer then the direct path, not only signal strength, but also
    received signal quality is affected (delay spread). In this case
    you will find few places with reasonable reception, even under good
    signal strength.

    Given your equipment (probably Frequency Modulated system with one
    receive antenna), a directional receive antenna may give improvement.
    The disadvantage is that you need a second person to have the antenna
    aimed at the transmitter (in case of moving setup).

    If this all is not possible and it must be wireless, you have to
    divert to multi antenna systems (space diversity) in combination with
    channel equalization or real wide band systems based on OFDM or other
    multi path resistant modulation scheme.

    Hope this helps you a bit.

    Wim
    PA3DJS
    www.tetech.nl
    please remove abc from the address in case of a direct reply.
     
  3. Mark

    Mark Guest

    yes it seems so...I have seen the same thing with those devices.

    it is very difficult to solve, you would need better antennas...

    in your case it may be more practical to use a coax wired connection

    For electrical saftey around water...be VERY careful that all
    equipment is properly grounded, that the outlet grounds are wired
    properly and I would also use GFI circuits.

    Mark
     
  4. Keith M

    Keith M Guest

    Well that's the first problem. Those cheap video transmitters are
    barely worth what you paid for them. There is a huge quality and price
    difference between models. Pay $50 and you get $50 worth of video quality.
    Ok, so not really a perfect environment for wireless. :)
    Get a better unit. Can you possibly run cable? Either use a better
    grade of video cable, or use a video-over-copper extender if the cable
    path is pretty long. Oh, and stay away from cheap video baluns too.
    If you have true line of sight, you are probably ok with the power.
    Is this really something you can change? I mean more than just a
    frequency within a range? If you can change bands, then maybe --- that
    usually takes dual-radios etc.
    The other poster Wimpie is right on. If you think it's multipath, then
    either use antenna diversity by using two antennas (which of course your
    radio has to support), or try using a directional antenna with a tight
    beamwidth. The stronger (higher dbi gain) antenna will have a smaller
    horizontal/vertical beamwidth. And so the chances of multipath will be
    lessened.

    Keith
     
  5. amdx

    amdx Guest

    No beanie but, http://www.detroitatvrepeater.com/rcplane.htm
    Mike
     
  6. mpm

    mpm Guest

    Are you holding the transmitter while you're moving?

    I am picturing you walking along the side of the pool as the kids swim
    from end to end.
    It's quite possible you are detuning the transmitting antenna when you
    step.

    This could be electromagnetic coupling, capacative, or who knows
    what??.
    The fact that you can stand still and get a good picture is a clue. -
    one that would seem to eliminate the multi-path thoeries....

    I generally agree you get what you pay for. But at 1.2GHz, these are
    going to be very small wavelengths. My first inclination is that this
    is not a multipath problem, though it is impossible to tell without
    further experimentation, and knowing a lot more about exactly what
    type of modulation scheme is being used.

    I second the thought about electrical safety around kids and water.
    -mpm


    .
     
  7. mpm

    mpm Guest

    Oops, another thought just came to me.
    If you're moving the camera / transmitter, it might have a real
    chicken-s^it timebase (crystal?) or whatever and could be sensitive to
    motion and going slightly off frequency with every bump (step taken).
    Maybe you can eliminate this possibiliity by walking very softly.
    Like that Karate guy on TV used to do on the onion paper... :)
     
  8. It is almost certainly multipath: The frequency, mobility, and
    environment are wrong for what you're trying to do. If you can lower
    the frequency - the lower, the better - then this will help. For
    example, 72 MHz. You an make it legal by applying for an
    experimenter's permit (if in the USA, see FCC form 442).

    I'm not sure if you can find any commercial transmitters below 900
    MHz, so you may need to DIY.

    There may be wireless products for DVB-H, DVB-T which would
    theoretically be an improvement.

    As one poster mentioned, your best bet may be a cable.

    Frank Raffaeli
     
  9. PhattyMo

    PhattyMo Guest


    What about a plain old modulator?
    They make some decent frequency agile ones for whole-house distribution
    and whatnot,some have up to ~1W of output. (CATV modulators)
     
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