[OFARC] Should Dipoles Made With Insulated Wire Be Made Shorter?

[N5DWI]

Planning to put up yet another dipole,
I decided to check the literature for anything new.

I found this:  I hope it helps someone.
73, John, N5DWI
=======================================

A new ham on the HamRadioHelpGroup mailing list recently asked about
what type of wire to use for building dipoles. There was a lot of talk
about what the guy should use, including copperweld, FlexWeave (my favorite),
and plain, old, 14-ga. insulated wire from one of the big-box hardware stores.

At that last recommendation, someone trotted out the old maxim that
if you use insulated wire, you’ll have to make the antenna shorter,
as the velocity factor of insulated wire is lower than the velocity
factor of bare wire. This factoid has been part of ham radio lore
for a long time. Is it true, though?

Well, not wanting to propagate (pun intended) an untruth,
I decided to Google this. The second result was a posting by Reg, G4FGQ,
titled Velocity factor of an insulated wire” on radiobanter.com.

Here’s what he posted:

    First calculate P = Q / ( R + S )

    where

    Q = K * Ln( 4 * H / d )
    R = K * Ln( 4 * H / D )
    S = Ln( D / d )

    and

    K = permittivity of insulating material.
    H = height of wire above ground.
    D = Diameter over insulating material.
    d = wire diameter.

    All dimensions in same units.

    Now P is the ratio of wire self-capacitance with insulation,
    to wire self-capacitance without insulation. It follows that –

    Velocity Factor = sqrt( 1 / P ) along the insulated wire.

    With ordinary antenna wire insulated up to twice the wire diameter
    with PVC, the reduction in velocity from the speed of light is
    insignificant and pruning can be forgotten about. It is much less than
    the pruning sometimes done for end effect which is usually unnecessary anyway.

    To detect the effect precision measurements are necessary unless
    the insulation has a diameter of several inches or more.
    For the permittivity of the insulation to have full effect
    it would be necessary to completely fill the infinity of space
    with the insulating material.

    So, it would appear that another old ham radio operator’s tale
    has been debunked.

    That other old hack, L(ft.) = 468/f(MHz) still rules. Have fun making dipoles.

============

    NOTE: I tried e-mailing G4FGQ to get permission to use his post on my blog here.
    Unfortunately, neither the website URL or e-mail address on his QRZ.Com page  are working. If you know how to get hold of him, please e-mail me with the details.

============

    Unfortunately, G4FGQ, passed away on Aug 29, 2006.
    K3HRN is maintaining his site information.
    Here is the URL: http://www.zerobeat.net/G4FGQ/

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