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Thread: Velocity factor vs. antenna length

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  1. #1
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    Default Velocity factor vs. antenna length

    If our concern is antenna size, why not play with the velocity factor in order to shorten up the antenna?

    How much shorter is a dipole made from insulated wire vs. bare?

    The velocity factor of insulated wire is between .6 and .7, but I don't think we can shorten up the antenna that much....or can we?
    73
    Mark, K8MHZ

  2. #2
    W5RB Guest

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    Quote Originally Posted by K8MHZ View Post
    If our concern is antenna size, why not play with the velocity factor in order to shorten up the antenna?

    How much shorter is a dipole made from insulated wire vs. bare?

    The velocity factor of insulated wire is between .6 and .7, but I don't think we can shorten up the antenna that much....or can we?
    No , it's not . No we can't .

  3. #3
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    Quote Originally Posted by W5RB View Post
    No , it's not . No we can't .
    This guy says different:

    http://www.hamuniverse.com/10metercoaxdipole.html
    73
    Mark, K8MHZ

  4. #4
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    Default

    Quote Originally Posted by K8MHZ View Post
    Will it work? yes! The same way a mobile antenna works, or one of those all band no-tune verticals from your favorite antenna manufacturer will work.

    This "10 meter coax dipole" is a compromise. How much? I will let someone with a more definitive knowledge give you that number

  5. #5
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    Quote Originally Posted by KE7HQY View Post
    Will it work? yes! The same way a mobile antenna works, or one of those all band no-tune verticals from your favorite antenna manufacturer will work.

    This "10 meter coax dipole" is a compromise. How much? I will let someone with a more definitive knowledge give you that number
    Coil loading works because of a change in velocity factor? I didn't know that.

    All antennas are compromises.

    I think the downside will be a decrease in gain, but there should also be an increase in bandwidth due to the larger diameter of the coax.

    I can think of a couple applications for an easy to make shortened antenna.

    So far, the lowest velocity factor in conductors is about .66. I wonder if there is a way to reduce that number by using different insulating materials.

    Usually the goal is just the opposite, so there may be a lack of experimentation because of that minor detail.
    73
    Mark, K8MHZ

  6. #6
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    Quote Originally Posted by K8MHZ View Post
    Coil loading works because of a change in velocity factor? I didn't know that.
    Should be careful with my analogy- by "the same way", I meant as a severe compromise antenna. Even wet spaghetti radiates


    EDIT: You might find these articles somewhat enlightening to the "gotchas" with this type of antenna, and how they "really" work.

    http://vk1od.net/antenna/PY2RML/
    http://www.karinya.net/g3txq/coax_antennas/
    Last edited by KE7HQY; 01-02-2011 at 09:36 PM.

  7. #7
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    Quote Originally Posted by K8MHZ View Post
    Usually the goal is just the opposite, so there may be a lack of experimentation because of that minor detail.
    And at the bottom of one of my linked article lies this message:

    Use at your own risk, not warranted for any purpose. Do not depend on any results without independent verification.
    Experiment away!
    Last edited by KE7HQY; 01-02-2011 at 09:40 PM.

  8. #8
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    Default

    Quote Originally Posted by K8MHZ View Post
    If our concern is antenna size, why not play with the velocity factor in order to shorten up the antenna?

    How much shorter is a dipole made from insulated wire vs. bare?
    Typically 4% shorter.

  9. #9

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    Quote Originally Posted by K8MHZ View Post
    So far, the lowest velocity factor in conductors is about .66. I wonder if there is a way to reduce that number by using different insulating materials.
    If you explore the effect of dielectric permittivity in a transmission line, yes, the velocity of wave propagation is slowed. When ALL of the electric field is immersed in a dielectric with relative permittivity εr, then vf= εr^-0.5.

    In the case of a PVC insulated antenna wire a long way above ground MOST of the dielectric is air (εr~=1), and a very small layer around the conductor is PVC (εr~=2.6). The combined effect is an average relative permittivity barely greater than 1, so the vf is almost 1.

    The notion that a PVC insulated antenna wire well above ground has a velocity factor anything much less than 1 is plain wrong.

    There are many designs for shortened antennas relying upon a flawed concept of velocity factor of the radiator, and curiously, Ham Universe publishes quite a few. You might form the view that Ham Universe gives harbor to flawed concepts.

    NEC4 can model the effect of insulation on an antenna wire, and various fudges are available to attempt the same thing in NEC2.

    Owen

  10. #10
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    What Owen said.

    In practice, I've found that insulated wire reduces the overall length of an HF dipole by a few inches, at most.

    73,
    Bill
    Member: Backyard Engineering Group

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