Calculating Coax Length by using a NanoVNA as a Time-Domain Reflectometer

Earlier in the month we posted about the NanoVNA, an open source VNA project by @edy555 and ttrftech that has recently become extremely affordable at US$50 for a fully assembled unit thanks to Chinese manufacturing.

The NanoVNA comes with preinstalled software for it's LCD GUI, as well as a Windows program. However, the software is currently basic and doesn't implement everything possible with a VNA. Over on his blog, nuclearrambo has put up a post showing how to use the NanoVNA as a Time-domain reflectometer (TDR). A TDR allows you to measure coax cable length, and that can be useful for finding exactly where a suspected cable or connector fault is.

Nucclearrambo provides a Python script which can be used with the NanoVNA's S1P output data to create a TDR graph. His tests with RG405 and LMR200 cable show that the length reported by the script comes remarkably close to the actual length.

In addition to the above, Ohan Smit has extended on nuclearrambo's work by modifying the C# NanoVNA Windows software (which was reverse engineered by Roger Clark) to automatically run the TDR script when S1P data is saved. Future work could see the Windows program support TDR by default.

Using the NanoVNA as a Time Domain Reflectometer.
Using the NanoVNA as a Time Domain Reflectometer.

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