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Coax with unknown length and unknown velocity factor?


Stan Ham 2021/01/04 09:57

I have the SAA2N from R&L Electronics, and am new and green to the nanoVNA's.  I am hoping one of you EEs out there can offer a solution for me.  I installed several years ago, a run of coax going from my shack, through walls, across an attic, and finally ported to a header outside.  I do understand that you must know either the velocity factor, or the electrical length to get the other's value.  I was wondering if there is a way to take another length of coax of known length and known velocity factor (or measured with the VNA), add it to this unknown coax, use the VNA to then measure the over-all values, and then calculate the length and velocity factor of the unknown coax.  Or, is there another method, either more or less complex than this idea?  Kind regards, Stan Ham, WB9GFA.

Siegfried Jackstien 2021/01/05 12:30

hmmm .. you want to get BOTH unknown values?? ... measure cable x ...
add known cable and measure cable x plus cable a ... and then do the
math to get cable x data ???

hmm tricky thing and i guess it will not work

in any case you measure only the electrical length ... and measured
result is electrical length = mechanical lenght divide / vf

you still will have two unknown values in the unknown cable ... be it
with or without extending with a known cable

if you have a length of exactly the same cable ... ok then you can
measure that short run (maybe a jumper in your shack made with same
cable years ago?!?) ... then you know mechanical and electrical length
(and so also vf is known) ... and then you can measure your cable from
shack to outside ... but without having a similar cable (to use as a
reference) its difficult to guess the mechanical length with measure the
electrical length

hope you understand now what its not easy!?!?!

greetz sigi dg9bfc

Am 04.01.2021 um 17:57 schrieb Stan Ham:

Solder Soldier 2021/01/05 05:30

Cut off a few feet, make your measurements and splice it back in. Not the neatest, but doable.
--
Solder Soldier

Alan “W2AEW” Wolke 2021/01/05 05:49

Since you installed it, do you have any left over?  Or, can you expose some of it?  If you do/can, simply look for the "type" of coax it is, usually printed on the outer jacket.  If you know the type (RG-8X, RG-58U, etc.), then you can easily look up the velocity factor.

Glenn Elmore 2021/01/05 06:15

Stan,
I might ask "why do you care?". If/since you know the electrical length and insertion loss what advantage do you get knowing the velocity factor, the ratio of electrical/physical length?
Glenn n6gn

ok1vaw 2021/01/07 02:06

Maybe looking at the type of insulation (dielectric) used in the coax could give a simple answer (or at least estimate), what it is and which velocity factor could you await. From the typical used types (polyethylene, PTFE, foam or similar)

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