WOW....that's exactly what I was going to say! :-)
David, K4ZZR
> At 7:53 PM -0400 10/3/02, K5RC wrote:
> >One problem.... 468/f does not work with insulated wire. You have to
> >consider the velocity factor of the jacket.
>
> Yes and no; but mostly no. Coaxial cable has a velocity factor
> significantly less than one, and equal to one divided by the square
> root of the dielectric constant of the insulating material that fills
> the space between the center conductor and the shield, because 100%
> of the electric stored energy of the wave(s) is stored in that space,
> in that insulating material. In other words, the capacitance per
> unit length of the transmission line, which together with the
> inductance per unit length determines the velocity of a wave
> traveling along the line, is proportional to that dielectric constant.
>
> With an insulated antenna wire in air or free space, typically the
> thickness of the dielectric/insulation is smaller than the diameter
> of the wire conductor and much, much smaller than the distance from
> the wire to the ground or other part of the antenna where electric
> field lines leaving the wire terminate. Most of the electric stored
> energy is *not* stored in the insulating material. The capacitance
> per unit length of the wire, which together with its inductance per
> unit length determines the velocity of a wave traveling along the
> wire, is only in small part determined by the dielectric constant of
> the insulation. Per unit length, the capacitance of the wire is that
> of two capacitors connected in series: the first being the
> coaxial-cylindrical capacitor formed by the insulation; and the
> second being the capacitance between the outer surface of the
> insulation and the ground or other part of the antenna where electric
> field lines leaving the insulated wire terminate.
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