[TowerTalk] Need help estimating cable length of DXE 400 MAX

[Jim Brown]

On 1/12/2020 11:07 AM, Michael Tope wrote:
> The published velocity factor for this type of cable is 0.81 so,
> 
>     Lp = 3514 ft * 0.81 = 2847 ft
> 
> 
> This is very close to what the guy was advertising, so I knew I wasn't 
> being short changed.

BUT -- VF for ALL transmission lines varies with frequency, and is 
typically about 5 percent slower in the range where you measured. The 
published value is that to which it converges at VHF, and is what most 
TDR instruments measure. The difference will make the line "measure" 
that percentage longer than it really is using the technique of looking 
for the first quarter-wave null.

See page 8 of http://k9yc.com/Coax-Stubs.pdf for an example of this 
measurement made by finding nulls at harmonic frequencies and plotting 
VF from those data. This can be done with instruments as simple as an 
MFJ-259 -- you turn the dial to find the null(s), then write down the 
frequency(ies) and compute VF.

73, Jim K9YC