Distance to fault (DTF) is how professionals find the exact spot where a coax cable is damaged — without climbing the tower or pulling the cable out of the wall. A water-filled connector, a crushed section under a clamp or a sharp kink shows up as a peak on a graph, with the distance in feet or meters. This article explains how distance to fault works, what you need to measure it, and how to read the result.
How distance to fault works
A VNA or cable-and-antenna analyzer sweeps a wide range of frequencies into one end of the cable and records the reflection (S11). An inverse Fourier transform turns that frequency data into reflection versus time — and, with the cable’s velocity factor, into reflection versus distance. Every impedance change in the line reflects part of the signal and appears as a peak.
Distance = (time × speed of light × velocity factor) / 2
Resolution ≈ (1.5 × 10^8 × VF) / sweep bandwidth [meters]The factor of 2 accounts for the round trip. A wider sweep gives finer resolution: with 3 GHz of bandwidth and VF 0.66, two faults about 3–4 cm apart can be separated.
What you need
- A VNA or analyzer with complex S11 data (NanoVNA, Siglent SHA850A and similar).
- The cable type, so you know its velocity factor and loss.
- A 50 Ω load or the antenna at the far end. An open far end is used for measuring length instead.
| Cable | Velocity factor | Loss at 1 GHz |
|---|---|---|
| RG-58 | 0.66 | about 0.6–0.7 dB/m |
| RG-174 | 0.66 | about 1 dB/m |
| RG-316 | 0.69–0.70 | about 0.9 dB/m |
| LMR-400 | 0.85 | about 0.13 dB/m |
| RG-213 | 0.66 | about 0.25 dB/m |
Reading a DTF graph
- Peak at zero — your own connector or adapter. Normal.
- Peak at the known cable length — the far-end connector or antenna. Expected.
- Peak in the middle with return loss worse than about 20 dB — a fault. Go there first.
- Rising background along the cable — water ingress or a damaged jacket over a long section.
- Nothing after a big peak — an open or short; the signal never gets past it.
DTF in SparSIT LAB
SparSIT LAB has a dedicated Cable page for Siglent analyzers. Pick the cable type from the built-in library (VF and loss fill in automatically), set the maximum distance, and press F5. The program:
- picks the sweep frequencies to cover the distance you need;
- computes DTF from complex S11 with a Hamming window, or uses the instrument’s own DTF mode;
- lists every discontinuity with its distance, return loss and a severity label (“moderate” or “strong”);
- reports “cable is uniform” when nothing is found.
The same page also measures insertion loss with both ends connected, one-port loss with a short at the far end, and cable length from the S11 phase slope.
Find coax faults in minutes
SparSIT LAB turns a Siglent analyzer into a DTF tester with a cable library, automatic fault list and test reports. Free beta for Windows.
Get free beta accessFAQ
Can a NanoVNA do distance to fault?
Yes, with PC software that performs the time-domain transform (for example NanoVNA-Saver’s TDR). Resolution is limited by the maximum frequency of your NanoVNA model.
Why is my distance wrong?
The velocity factor is wrong. Measure a known length of the same cable, adjust VF until the distance matches, and use that value.
What return loss means a bad connector?
As a rule of thumb, a good connector on a new cable shows better than about 25–30 dB. A peak at 15–20 dB deserves a closer look; below 15 dB something is clearly wrong.
For background on coax impedance and loss, see coax cable basics (in Ukrainian, auto-translated).
Sources: Siglent: what is DTF measuring.