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Velocity factor vs. antenna length

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by K8MHZ, Jan 2, 2011.

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  1. K8MHZ

    K8MHZ Ham Member QRZ Page

    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?
     
  2. W5RB

    W5RB Guest

    No , it's not . No we can't .
     
  3. K8MHZ

    K8MHZ Ham Member QRZ Page

  4. AD7N

    AD7N Ham Member QRZ Page

    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 :D
     
  5. K8MHZ

    K8MHZ Ham Member QRZ Page

    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.
     
  6. AD7N

    AD7N Ham Member QRZ Page

    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: Jan 2, 2011
  7. AD7N

    AD7N Ham Member QRZ Page

    And at the bottom of one of my linked article lies this message:

    Experiment away!
     
    Last edited: Jan 2, 2011
  8. M0GVZ

    M0GVZ XML Subscriber QRZ Page

    Typically 4% shorter.
     
  9. VK1OD

    VK1OD Ham Member QRZ Page

    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. K7JBQ

    K7JBQ Moderator Volunteer Moderator QRZ Page

    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
     
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