Cable fault distance calculator
This is what people buy an analyser for
A cable in a conduit or under a roof cannot be cut in half to check it. But the reflection from a fault comes back with a delay, and from that delay the place can be found to within half a metre — with a fifty-dollar instrument.
| Measurement method | |
| Cable type | |
| Velocity factor vf | |
| Spacing between resonances (MHz) | |
| What the sign of the reflection tells you | |
| Result | |
| Distance to the reflection point | |
📐 The formulas
Resonance spacing (NanoVNA in S11 mode):
d = c · vf / (2 · Δf)
Reflection delay (oscilloscope or TDR):
d = c · vf · t / 2
S11 phase at a single frequency:
d = (φ / 360°) × λ × vf / 2
The division by two is because the signal covers the distance twice, there and back.
What the sign of the reflection tells you:
| Reflection | Cause |
|---|---|
| Γ positive, |Z| rises | open circuit |
| Γ negative, |Z| falls | short circuit |
| |Γ| noticeably below 1 | a kink, water, a bad connector |
Where this calculator stops:
- The phase method is ambiguous: the same phase repeats every half wavelength in the cable. The calculator shows the ambiguity step
- The velocity factor comes from a table; it is safer to measure it on an intact length from the same drum
- Water in the cable smears the reflection along its length, and then there is no exact distance to be had
- Several faults at once turn the picture to mush: the method sees the first one
🎯 In practice
- Water in the feedline up the mast. The commonest case and the most useful answer.
- A break in a cable buried in a wall.
- Checking the length of cable on a drum without unwinding it.
Practical advice:
- Leave the far end open while measuring, so the reflection from the end is distinct from the fault.
- If the distance comes out exactly twice what you expected, you forgot to divide by two.
- Measure a known intact length first: that shows immediately whether your vf is right.
- A bad connector gives the same reflection as a cable fault. Start by checking the connectors.
© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.
How to cite this calculator
UR3PKI. «Cable fault distance calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/lines/tdr (licence CC BY-NC-SA 4.0).The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.