You’ve got several spools, coils, and runs of
coaxial cable lying around your garage, with
only a rough idea of how long each is. You
keep telling yourself that someday you’ll get
around to laying them out and measuring
how long they are, so you’ll know just what
you have. But what a pain.
You can always measure the coil diameter,
multiply by pi, and by the number of coils,
and come close, but that estimate might have
bitten you a couple of times.
Turns out you could use your NanoVNA to
measure those lengths, saving a little time
and trouble, while getting a more accurate
estimate. It does help to have an RF connect-
or on at least one end of your coax, so you
can easily connect it to your NanoVNA.
Getting set up
First, find out what model of coax you have,
so you can determine its velocity factor. Here
are some common ones:
Coax Model Impedance Velocity Factor
RG-8X 50 ohms 82%
RG-58 50 ohms 66%
RG-59 75 ohms 66%
RG-8 50 ohms 66%
RG-6 75 ohms 66%
RG-213 50 ohms 66%
LMR-400 50 ohms 85%
Connect your coax to the CHØ port of your
NanoVNA (labeled S11 or TX on some mod-
els), leaving the CH1 port disconnected. Turn
on the NanoVNA.
The Steps
Tap DISPLAY, then TRACE
Disable all but TRACE 0, then tap BACK
Tap CHANNEL, then CHØ REFLECT
Tap FORMAT, then MORE, then LINEAR
Tap BACK, then BACK
Tap TRANSFORM
Tap LOW PASS IMPULSE
Tap TRANSFORM ON
Tap VELOCITY FACTOR and enter the VF in
whole numbers (like 66), then x1
Tap BACK, then BACK, then STIMULUS
Tap START and set it to 50K
Tap STOP and set it to some guess from
200M to 240M (start with 200M), then BACK
Tap MARKER, then SEARCH, then MAXIMUM
The NanoVNA will display the response graph
of the coax, indicating a “peak” at a certain
distance from the start (left end). In this case,
it shows my coax as 9.13 meters long, which
is about (9.13 m x 39.37 in/m ÷ 12 in/ft ≈)
30 feet long. For extra credit, if my coax had
a break in it somewhere, the first peak would
be the location of the cable break.
Acknowledgement: Noji Ratzlaff, KNØJI (kn0ji@arrl.net)
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