On Thu, Jun 8, 2023 at 11:41 AM, Stan Dye wrote:
A far easier and more accurate way to measure cable length is by using the
"Measure -> Cable" function in the nanovna firmware (if you have a firmware
with this function).
Just connect to an open-ended cable, and read the length and cable loss on
the left side of the screen. You still have to set the correct VF, of
course. But this method does not use TDR and transforms, with the
associated complications. Instead, it finds the quarter-wavelength
frequency of the cable by finding the first S11 phase reversal, and does
the simple calculation.
Stan,
I have to disagree with you on this one. The method uses the 1/4 wavelength measurement method but it is prone to more error that the TDR method. Here are some actual measurements to illustrate my point...
I used the same test cable as in my previous post. This was a 9.38 meter long RG58 A/u cable with a measured 64% VF using a pulse/scope TDR technique. Using NanoVNA Saver the calculated value was 9.34 meters. Using the NanoVNA transform yielded 9.33 meters. These are both 5 cm (about 2 inches).
Using the Cable Measure feature the length depended on the calibrated frequency range. Here are the results with various frequency ranges and an open or short BNC cal terminations on the end. VF set to 64%. Screenshots attached.
50 kHz to 900 MHz. 9.240 meters (open) 9.331 meters (short)
50 kHz to 100 MHz. 9.586 meters (open) 9.608 meters (short)
50 kHz to 20 MHz. 9.588 meters (open) 9.610 meters (short)
One can clearly see that the results are much worse than the TDR method in Saver and on the NanoVNA
Looking at the screenshots you can see the following. The 1/4 wavelength frequency is about 5 MHz.. VF varies with frequency (see attached plot) and does not level off to the "nominal VF" published by the manufacturer until you are much higher in frequency. So this 1/4 wavelength method is subject to the user inputting a VF which is not correct for the 1/4 wavelength measuring frequency. The problem gets worse for longer cables because the 1/4 wavelength frequency is even lower! The second issue is that the user has to calibrate for a frequency range that does not result in too much interpolation which is what happened for the 50 kHz. to 900 MHz. measurement posted above.
In summary the Cable measure function in the nanoVNA requires fewer setup steps but is prone to greater error because the VF that is entered is different than the nominal published VF or one measured using a pulse/scope or TDR instrument. Users need to be wary of using a feature without know how it works or its limitations.
Roger
Roger Need
