[TowerTalk] velocity factor?

[Jim Brown]

On Wed,10/8/2014 10:19 AM, Hans Hammarquist via TowerTalk wrote:
> I decided to also check the velocity factor. I was a little puzzled to get a factor of 0.8. Does that make sense? It's a foam filled coax. I expected something around 0.9.

A couple of comments. First, Vf varies slightly with frequency 
(typically 1% between 2 MHz and 50 MHz. How did you measure Vf? 
Measuring the resonant frequency of the line as a stub will yield the 
value at at that frequency. Measuring TDR by doing an FFT of a VHF sweep 
will yield the VHF value.

Second, all foam dielectrics are not the same. I have foam dielectric 
coaxes with values between about 0.78 and 0.88 at VHF.

I suggest you dig up the mfr data sheet for the coax you have. The 
published Vf will be the VHF value.

There's a discussion of this in k9yc/com/Coax-Stubs.pdf

73, Jim K9YC