On Thu, Jun 8, 2023 at 04:34 AM, Bryan Curl wrote:
I always had this question as well
Since you must enter a VF it must be calculating physical length.
Things that effect accuracy are stop freq. I got better accuracy when the
indication filled the graph. I adjust the stop to get best fit then
recalibrate. Also setting it up to do 401 pts for the sweep improved accuracy.
Still its just an estimate.
The NanoVNA does not calculate the physical cable length using traditional Time Domain Reflectometry (with pulses on the cable). It calculates using frequency domain S11 data and then performs an Inverse Fast Fourier Transfom (IFFT) and uses the velocity factor VF (input by the user) to calculate the physical length. It can be quite accurate if you know the VF and select an appropriate frequency range. Below are some tests I completed using NanoVNA Saver on a 9.38 meter (30.75 foot) piece of Beleden 8259 RG-58A/U cable. In the first test I selected a cable type that was similar from the drop down menu and Saver calculated the physical length as 9.628 meters (31.6 feet) using a VF of .66. I measured the actual VF using an analog TDR system and the VF for this cable was .64. I set this value in Saver using the custom option from the drop down menu and Saver calculated 9.34 meters which is very close the the actual cable length of 9.38 meters.
The frequency range used in my tests was 50 kHz. to 900 MHz. and I used the Manage button in Saver to select 401 data points for the measurement. This works well for cables up to 30 meters in length. For longer cables you need to reduce the stimulus frequency range. For much shorter cables the stimulus range should be higher if you want a better estimate.
Roger
Roger Need
