Connect with us

antenna length for video xmtr

Discussion in 'Electronic Basics' started by Eric R Snow, Sep 17, 2004.

Scroll to continue with content
  1. Eric R Snow

    Eric R Snow Guest

    Greetings Folks,
    I have a video camera/transmitter unit that transmits in the 900 MHz
    to 1200 MHz range according to the spec sheet. It currently has a
    wire for the antenna that's 3.5 inches long. For optimum transmitting
    doesn't the antenna length need to be a length that divides evenly
    into wavelength? I will be enclosing the camera in a small plastic
    waterproof enclosure with batteries and a light. Then it will be
    lowered 70 feet down a 6 inch diameter iron pipe. It will be used to
    inspect the well casing and pump as well as to watch the water as it
    is being pumped. I want the best signal possible and was wondering how
    long to make the antenna if the present one doesn't work. Right now,
    in open air, a good signal is present at 100 feet. But I don't know
    how well it will work when lowered down the iron pipe.
    Thank You,
    Eric R Snow
     
  2. Bob Myers

    Bob Myers Guest

    If you're going to be lowering the thing down
    an iron pipe, I would think you would do FAR
    better simply running the signal over a cable than
    you ever will with a wireless link.

    Bob M.
     
  3. Eric R Snow

    Eric R Snow Guest

    That's what I'm afraid of. But it's much easier to keep the camera dry
    and to maneuver it around if it's only connected by a rope. So I'm
    gonna try it first.
    ERS
     
  4. Bob Myers

    Bob Myers Guest

    There's a problem with the "rope" being a coaxial
    cable, or at least having a mini-coax wrapped around
    it?

    You're going to have REAL problems with an RF link
    in this case, trust me...


    Bob M.
     
  5. Si Ballenger

    Si Ballenger Guest

    You need to run your video and power over a cable (easy to do
    using 4 conductor telephone wire). The below cams would be easy
    and cheap to use with a little tweeking/modding.

    http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=91309
    http://www.harborfreight.com/cpi/ctaf/Displayitem.taf?itemnumber=47546
     
  6. Eric R Snow

    Eric R Snow Guest

    Yeah Bob, I tried the cable as rope thing and the cable wrapped around
    the rope and had problems with manipulation and strength problems. I
    guess I'll lower tthe camera down and if there are problems then go to
    the extra trouble to make a case with sealed leads coming through. .
    ERS
     
  7. Captain

    Captain Guest

    1/4 wavelength is the best length. L=C/F (C=3E8) For 900 MHz this is 333
    cm. Cut the antenna a bit longer and trim until you get best signal
    strength. If it will fit, silver plated narrow gauge welding rod works
    well.

    Actually, with the transmitter down a metal pipe you could get a wave guide
    effect, in which case the signal will boom out.

    Cap
     
  8. ^^ ?

    No, 3e8/9e8 = 1/3m. Thats the wavelength in meters.

    1/4 of that is 8.33cm, or 3.28 in. Your antenna is already close to 1/4
    of a wavelength.
    Regards,
    Bob Monsen
     
  9. Eric R Snow

    Eric R Snow Guest

    Greetyings Bob,
    It's interesting that the calculations comes so close to the length.
    Since the thing transmits anywhere in the 900 to 1200 range the
    antenna being 3.5 inches long would seem to be covering that range.
    If the antenna needs to be extended and a shielded wire is used until
    the last 3.5 inches of wire will the effect be the same as the antenna
    being connected directly to the circuit board as it is now?
    Thanks,
    eric
     
  10. Rich Grise

    Rich Grise Guest

    What strength problems? If the rope will hold the camera without
    the cable, what makes the rope becomes weaker when there's a cable
    dangling alongside it? You leave some slack at the camera end, and
    let the cable support only its own weight. You could keep them at
    the same length with some kind of capstan.

    Good Luck!
    Rich
     
  11. Captain

    Captain Guest

    Mea culpa! I thought a foot was a bit long for the frequency!
    Cap
     
  12. Eric R Snow

    Eric R Snow Guest

    Rich,
    The video cable by itself was too weak. Wrapped arond the rope it was
    a mess.
    ERS
     
  13. Si Ballenger

    Si Ballenger Guest

    How much weight are you trying to drop down your well? Most cams
    small enough to drop down the well casing with the pump still
    installed weigh only a couple of ounces at most. Bullet types of
    cams usually have a threaded fitting on the rear in which a ~1'
    length piece of threaded rod can be attached. Tape the cam cable
    to the threaded rod so all hangs vertically. You can see the
    threaded hole on the rear of the little IR cam on the below page
    (this cam is small, but the bullet cams are even thinner). The
    cable with this cam probably could withstand a 50lb pull if it
    got stuck. The rear of the cam would need to be tapered to the
    threaded rod so the edge wouldn't catch on a casing misalignment
    and such. Sounds like you are trying to put too large a cam setup
    down your casing.

    http://www.geocities.com/zoomkat/ezservo.htm
     
  14. Eric R Snow

    Eric R Snow Guest

    The camera is small. The casing with batteries and LED lights is the
    big part.
    ERS
     
  15. Cable TV companies use a cable consisting of a coax and a stronger
    support wire for the drops from a utility pole to a house - perhaps
    you could use that... (Belden 7983A, for example)





    --
    Peter Bennett, VE7CEI
    peterbb4 (at) interchange.ubc.ca
    new newsgroup users info : http://vancouver-webpages.com/nnq
    GPS and NMEA info: http://vancouver-webpages.com/peter
    Vancouver Power Squadron: http://vancouver.powersquadron.ca
     
  16. Si Ballenger

    Si Ballenger Guest

    I've only pulled a couple of well pumps, but I wouldn't be
    putting all that stuff down the well casing. You are just asking
    for trouble. Go to google search to see what the well inspectors
    use. An IR bullet cam on the end of cheap cat5 cable will be
    plenty strong enough to support the cam and pull back out if it
    gets in a bind. Below is a cam you might want to look into. YMMV!

    http://www.supercircuits.com/store/prodinfo.asp?number=PC136UW1&variation=&aitem=7&mitem=11
     
  17. Rich Grise

    Rich Grise Guest

    What part of "alongside" don't you understand?

    Thanks,
    Rich
     
  18. Rich Grise

    Rich Grise Guest

    http://www.supercircuits.com/store/prodinfo.asp?number=PC136UW1&variation=&aitem=7&mitem=11

    And leave the batteries topside, of course.

    Cheers!
    Rich
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-