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

    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

  4. AD7N

    AD7N Ham Member

    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

    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

    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

    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

    Typically 4% shorter.
     
  9. VK1OD

    VK1OD Ham Member

    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

    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
     
  11. G3TXQ

    G3TXQ Ham Member

  12. K8MHZ

    K8MHZ Ham Member

  13. W8JI

    W8JI Ham Member

  14. G3TXQ

    G3TXQ Ham Member

    Here's another HamUniverse article on the same subject, where the author gets it totally wrong:
    http://www.hamuniverse.com/17mcoaxmox.html

    Notice the parasitic Reflector element - the author thinks it can be reduced to 66% of its original length simply by making it out of coax - braid and inner connected at either end !!!!!!

    It's not 1st April is it?

    Steve G3TXQ
     
  15. W8JI

    W8JI Ham Member

    Ham Universe is full of things like that.

    Look at this antenna. All band Sterba 160-10 and a ground systemthat gives it 3-6dB gain:

    http://www.hamuniverse.com/sturba.html

    or this, a V-beam that is only 1/4 wave or less on a leg:

    http://www.hamuniverse.com/kl7jrhorizminibeams.html

    or this one element multi-band beam that with 12.33 foot legs has 2.1 dBd gain and 18 dB F/B on 20 meters. On the band where it is "almost a dipole" it has 2.8 dB gain over a dipole:

    http://www.hamuniverse.com/multivee.html

    or the one element dipole beam with 1/4 wave long sides that has 3 to 4 dB gain over a dipole because a dipole is bent into a V:

    http://www.hamuniverse.com/1elbeam.html

    Probably because of the better space filling caused by the fractal shape?
     
  16. M3KXZ

    M3KXZ XML Subscriber

    I wonder..... Tom, Steve et al..... have you tried emailing the fellow hams who have published articles these coax antennas to explain why their thinking might be flawed? Or is that not the done thing?
     
  17. AC0FP

    AC0FP Ham Member

    I think your confusing the velocity factor of a transmission line with that of insulated wire used in making an antenna.
     
  18. G3TXQ

    G3TXQ Ham Member

    No, is my answer.

    It's probably a "cop-out" to say it, but if I did that with every flawed web article I came across I would have time for nothing else. I don't know what the solution is, but it would be a start if sites such as HamUniverse subjected articles to rigorous peer review before they were published.

    Perhaps more worrying is the lack of rigorous review by some of the leading paper magazines; I guess many newcomers have learned to be wary of the accuracy of web articles, but they "take as gospel" what they read in the magazines.

    73,
    Steve G3TXQ
     
  19. W8JI

    W8JI Ham Member

    I have for several articles. There was no response other than one saying many people have built the antenna and were overjoyed. :) A fellow who worked for me built and sold an antenna made from coax, and when I showed him how it didn't actually work as he claimed he said "oh well, at least they sell" and continued selling them at hamfests. It's a strange world.
     
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