[Antennas] 1/4 Wavelength Coax

[Richard (Rick) Karlquist]

It's a trick question.  If you are using the
coax as a transmission line the second formula
is correct.  If you are using the coax as an
a form of wire to make a wire antenna then
you need the first formula. However, the
"velocity" factor for that formula is NOT
one the manufacturer lists for the coax
as a transmission line.  Rather it accounts
for the outer vinyl jacket of the coax acting
as a dielectric.  The number 234 is 5% less
than 246 to account for "end effect".

It might seem surprising that a fairly thin
layer of dielectric (much smaller than the
diameter of the outer conductor) would have
any substantial effect on velocity factor.
The reason why it does matter is that the
electric field is inversely proportional to
the distance from the center of the coax.
IE, it is highly concentrated at the surface
of the coax.  You can actually calculate it
if you know the dimensions and the permittivity
of the jacket material.

Rick N6RK



The first formula adjusts
by 5% to account for "end effect" and the
the dielectric

On 12/26/2015 11:38 AM, Dick Dickinson wrote:
> I'm attempting to fine tune some coax lengths for phased verticals.
>
> In searching the internet, I find two formulas used to determine a 1/4
> wavelength of coax.
>
> 	234 / Frequency x Velocity Factor, and
> 	246 / Frequency x Velocity Factor
>
>
> What constant is correct for coax lengths?
>
>
> 73,
> Dick - KA5KKT
>
>
>
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