[TowerTalk] Veloc Factors - wire + Lad line - from DAVIS RF Co.

[Jim Lux]

-----Original Message-----
>From: "Steve Davis -Davis RF Co." <sdavis at davisrf.com>
>Sent: Jan 5, 2008 4:10 PM
>To: towertalk at contesting.com, k7ddmjb at 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