开云体育

Some very important observations in previous responses:
1 - to phase match, you don't need any particular value, just to make them
the same for two cables
2 - at your target frequency, 5 degrees is about 1/10 of an inch, a very
exacting measurement over 20 feet of cable: just stress in the cable or
connector issues could easily change its electrical length by that much.
And the velocity factor variability in the cables could readily introduce
that much uncertainty.
3 - you do want to leave the far end of the cable open when using the
nanovna for measurements

Another important measurement issue:
The nanovna 'tdr' measurements can't measure within 0.1 inch at 20 feet.
This is because it is not a true TDR - it uses frequency scanning at
discrete frequency points, then an FFT to compute the corresponding
length. The 'bin' size of the FFT depends on the size of the frequency
scan steps (with an inverse relationship). In TDR mode you can see this as
you move the cursor across the display. The 'length' measurement will be
in steps of so many mm or cm. That is the finest granularity that the TDR
can do for the given frequency range. You can make it more accurate by
expanding the frequency span - but only to a point

One way to deal with this in your situation is to measure one cable,
trimming it by about 1/10 inch at a time just until the nanovna measurement
pops from one 'bin' to the next - meaning you are measuring at the edge of
that bin. Then do the same with the other cable, and you should be very
close. (Assuming there are not significant velocity factor differences in
the two lengths of cable).

Another perhaps better way is to not use the TDR mode, but (if your
firmware has it) the Measure / Cable function. Leave the cable open at the
end. Set the top frequency of the vna such that the smith chart trace is
just over a half-circle. Then note the length of the cable shown. Trim
the other cable to show that same length. You can ignore the velocity
factor and whether or not the length is correct to measured length, since
you only need them to match electrically. This method is usually more
accurate, because it measures the quarter-wavelength resonance of the cable
by finding the phase crossing, using excellent interpolation between
measured frequency points.

Good luck.

Stan