If you want to know the PHYSICAL length of coax,
measure the low frequency inductance rather than
the capacitance. The inductance will be independent
of dielectric properties. The physical length
can be calculated from the inductance and the
ratio of outer to inner diameter, which should
be shown on the data sheet.
9913 is mostly air, so the capacitance method
is almost as good as the inductance method.
However, it isn't as good for ordinary coax,
where the VF can vary considerably.
Rick N6RK
On 10/28/2014 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!
-Mike-WA6ZTY
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