[TowerTalk] Measuring Coax Length

[Jim Lux]

On 10/28/14, 2:15 PM, Mike Fahmie via TowerTalk wrote:
> Some years back, I purchased a 500' reel of Belden 9913 and used a bit here and there over the years.  Now I needed four 70' runs to the base of my tower but didn't know how much 9913 remained on the reel.
> My DVM can measure capacitance so I connected it to the free end of the reel and measured 6300 pF.  Belden says that 9913 capacitance is 24.6 pF/ft so dividing that into 6300 pF implied that I had 256 ft remaining.  That was enough for three 70' runs but not enough for four,  oh well.
> I cut three 70' lengths from the reel, 45' of cable remained.  (3 x 70) + 45 = 255.  That is just 1 ft less than what was predicted by the capacitance measurement !!!  Isn't it nice when the numbers work out so well.  Will I rely on that technique in the future?  Nope, gotta see it work out like that a few more times first!
>

You could also measure the C after you cut the first 70 foot chunk.  The 
measurement of the 70 foot chunk will be closer in C to the stuff on the 
reel, AND, it will be the actual C of YOUR particular lot of cable.

Pretty clever idea, though.

And basically about the same accuracy, I'd think, as the various TDR 
techniques.  For TDR (or looking for integer half wavelengths as you 
sweep the frequency on an analyzer) you have to assume a particular 
propagation factor, and that's affected by L and C as well. L isn't 
strongly affected by the physical size of the conductor: more the 
physical length, but C sure is affected by the dielectric AND the 
spacing of the shield/center, which is where the manufacturing 
variations probably are.