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diy uhf TV antenna

Discussion in 'Electronic Repair' started by [email protected], Mar 27, 2006.

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  1. Guest

    hi there,am tinkering with an old tv and would like to learn more about
    antennas,& try with different shapes. what i would like to understand
    before starting is if an antenna is basically a closed loop of wire
    shorting the inner pole and outer poles on the RF out on the back of
    the tv

    what should the resistance of this loop be in total?

    or is it an open loop?
    thx
     
  2. Guest

    1 beerismy:
    There is most likely NO RF out on the back of the TV.... it is
    probably an antenna or RF INPUT. If it is a "F" type coaxiall jack
    the nominal impedance will be 75 ohms, if you have two screws for the
    antenna or RF input, the nominal impedance will likely be 300 ohms.
    Note that the impedance is NOT the same as DC resistance like you would
    measure with your multimeter.
    A simple UHF antenna could be a stiff wire loop of about 7 inches in
    diameter. This will have a DC resistance of near ZERO ohms but an
    impedance of about 300 ohms at UHF frequencies so if you are connecting
    it to a 75 ohm input jack you will need a 300 / 75 ohm balun matching
    transformer for best performance.
    electricitym
    ..
    ..
    ..
     
  3. Andy

    Andy Guest

    Andy replies:

    Get a copy of the ARRL handbook or, even better, the ARRL
    Antenna Handbook...

    You have a lot of getting up to speed to do before you will
    actually accomplish anything. Once you understand the basics,
    you can do almost anything with coat hangers. Otherwise, you
    will only create bent up wires that may or may not work at some
    frequencies and not at others...

    It ain't black magic --- antenna design has very definite rules
    and they are not difficult. It's a lot like hanging sheet rock. If
    you understand what you are doing, it is easy. If you don't , you
    have a mess.... good luck...

    We all started just the way you are now. You may find that
    it is a hell of a lot of fun. Or you may decide, "the hell with it".
    But there's no reason for "trial and error" when the actual discipline
    is very methodical and reasonable....... about the complexity as
    changing the oil in your new truck for the first time....

    Andy W4OAH
     
  4. Guest

    Sometimes its an open loop, at least DC resistance. Sometimes its a DC
    closed loop.
    When its shorted, there will be near zero ohms DC resistance.
    In the world of RF and antennas, impedance has no bearing on the DC
    resistance.
    An antenna has a specific impedance over a certain range of
    frequencies, and
    the transmission line feeding the antenna must match the antenna
    impedance, as
    well as match the input impedance of a TV or other device. I started
    out with an ARRL antenna book and a ARRL handbook back in about 1962.
    Got my Novice class then.

    greg
     
  5. Guest

    Years ago I made a Yagi antenna with an aluminium square tube with
    bars screwed on with stanless steel screws.
    It had about 20 elements on it.
    But now I want to make another one and I don't have a plan any more.
    Is there any good plans available on the web?
    I must admit I have not even looked yet.
     
  6. Guest

    i have read a bit more on this and have successfully created a simple
    dipole which gets VHF & some UHF too.my favorite channels come in with
    no snow, but with some ghosting.

    i read that raising the dipole in the sky would cure it but would
    require building a tower.

    would adding a reflector help with this? i read that a dipole's
    reflector is a piece of wire a bit longer than the diapole. would it
    make any difference if i get more drastic and use a rectangle of foil
    or chicken mesh to block out everything coming from behind the diapole?


    thx
     
  7. GregS

    GregS Guest

    Rotating the dipole will help remove ghosting. I used to buy corner reflectors, hardly
    worth making your own. If they don't sell them, you will have to try doing what you
    said. If you found formulas for making yagis, the distance between and the length of
     ”ÿ
    ratio over a driven and reflector rod combination.

    greg
     
  8. GregS

    GregS Guest

    Repaired that post.

    Rotating the dipole will help remove ghosting. I used to buy corner reflectors, hardly
    worth making your own. If they don't sell them, you will have to try doing what you
    said. If you found formulas for making yagis, the distance between and the length of
    elements can be adjusted. You can increase the size of the back reflector as
    you mentioned, and will increase the front to back ratio further than a plain reflector
    eolement.

    greg
     
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