-----Original Message-----
>From: "Steve Davis -Davis RF Co." <sdavis@davisrf.com>
>Sent: Jan 5, 2008 4:10 PM
>To: towertalk@contesting.com, k7ddmjb@qwest.net
>Subject: [TowerTalk] Veloc Factors - wire + Lad line - from DAVIS RF Co.
>
>I was asked by Michael Baker, K7DD, to provide the VF's for various
>Flex-Weave Tm items. For the following reasons, there is no single answer
>for this question without multiple factors being considered, however, the
>"nominal" answer will certainly obtain optimal performance, short only
>perhaps to "rocket science":
>
>
>
>Propagation on bare, solid, or stranded copper or CCS (cop clad steel) wire
>in a vacuum yields a VF of 100% or 1.00, the speed of light.
>
>
>In air at sea level the nominal figure is 95%, used in dipole length
>calculations.
>In insulated wire, the figure varies around .90 - .95 - a good figure to use
>in calculations such as EZNEC, etc, since any solution will be affected by
>altitude, temperature, humidity, insulation thickness variance ,density, and
>age, plus ultraviolet radiation and plain old dirt film.
But this is not really a velocity factor. It's more of an empirically derived
number that makes a dipole have no reactive component (whether you want to call
it "end effect", "dielectric loading", etc.). Surrounding the wire with a
dielectric (be it polyethylene or just dirt and grunge) will change the
resonant length, but it's not something that is really a "velocity of
propagation" kind of thing (other than epsilon affects both the capacitance and
the propagation speed).
Even an infinitely thin dipole of perfectly conducting wire is resonant
(meaning no reactive term in feedpoint impedance) at slightly less than a half
wavelength.
And, the 0.95 thing is convenient (as Steve says, it's easier to cut or fold
back the extra than to add some on the end).
>
>
>If you are using the PE insulated versions of ladder line, either stranded
>or solid, 450 ohm having a few awg sizes, or the 300 ohm, over time we have
>found that .91 VF is the number.
Now those ARE transmission lines, and would have a velocity factor associated
with them.
Jim, W6RMK
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|