Time Domain Reflectometers
A time domain reflectometer (TDR) locates discontinuities in transmission lines — impedance mismatches, open circuits, short circuits, water ingress, and connector damage — by measuring the time for a reflected pulse to return. The principle is identical to radar: send a known pulse into the cable, measure how long it takes to return, and calculate the distance to the fault. Instead of electromagnetic waves bouncing off distant aircraft, you are bouncing fast electrical pulses off the inside of your coaxial feedline.
For amateur radio, TDR measurements answer questions that are otherwise very difficult: How long is this unlabeled coaxial cable? Where exactly is the fault in the 200-foot run of buried coax to the tower? Is the connector at the top of the tower good or intermittent? What is this cable's actual velocity factor? A dedicated TDR instrument costs $500–$5000. A NanoVNA with time-domain transformation capability (enabled in software) can serve as a functional TDR for many of these tasks at no additional cost.
TDR Principle
A TDR applies a fast-rising voltage step (or pulse) to one end of a transmission line and monitors the voltage at the same point over time. The signal travels along the cable at the propagation velocity of the cable. When it encounters an impedance discontinuity — a connector, a damaged section, or the far end of the cable — some energy reflects back toward the source. The TDR measures the time from the initial pulse launch to the arrival of the reflection. Half of this round-trip time is the one-way travel time to the discontinuity.
Distance formula:
d = (troundtrip × vp) / 2
Where d is the distance to the discontinuity, troundtrip is the measured round-trip travel time, and vp is the propagation velocity in the cable (= velocity factor × speed of light).
The speed of light in a vacuum is c = 3 × 10⁸ m/s ≈ 0.984 ft/ns. For a cable with velocity factor VF = 0.66 (such as polyethylene-dielectric coaxial cable), the propagation velocity is 0.66 × 3 × 10⁸ = 1.98 × 10⁸ m/s = 1.98 ft/ns × 0.984 = 0.649 ft/ns. In one nanosecond, the signal in this cable travels about 0.649 feet (0.198 meters).
Velocity Factor and the Speed of Propagation
The velocity factor (VF) of a coaxial cable is the ratio of signal propagation speed in the cable to the speed of light in vacuum. It is always less than 1 because the dielectric material between the inner and outer conductors has a relative permittivity (dielectric constant) greater than 1, which slows the propagation of the electromagnetic wave.
VF = 1 / √εr
Where εr is the relative permittivity of the dielectric. For solid polyethylene (εr = 2.25): VF = 1/√2.25 = 1/1.5 = 0.667. For foam polyethylene (εr ≈ 1.5): VF = 1/√1.5 ≈ 0.82. For air-spaced cable (εr ≈ 1.05): VF ≈ 0.975.
| Cable Type | Typical VF | Propagation Speed (ft/ns) |
|---|---|---|
| RG-58 (solid PE) | 0.659 | 0.649 |
| RG-8X (foam PE) | 0.78 | 0.768 |
| LMR-400 (foam PE) | 0.85 | 0.836 |
| RG-213 (solid PE) | 0.659 | 0.649 |
| Hardline (air/foam) | 0.88–0.93 | 0.866–0.915 |
| 450-Ω ladder line (air) | 0.91–0.95 | 0.895–0.935 |
Entering the wrong velocity factor into a TDR measurement is the most common source of large distance errors. Always use the velocity factor from the cable's datasheet or measure it yourself: cut a known length of cable, measure the TDR round-trip time, and calculate VF = 2d / (t × c).
Interpreting TDR Waveforms
The shape of the reflected pulse tells you the nature of the discontinuity:
Open circuit (cable end with nothing connected): The reflection is positive — the reflected wave has the same polarity as the incident wave. On the TDR display, the waveform steps up at the time corresponding to the cable end. This is the most common measurement — locating the end of an unknown cable length.
Short circuit: The reflection is negative — equal amplitude but opposite polarity to the incident wave. On the TDR display, the waveform steps down at the time of the short. This signature indicates a dead short in the cable — usually a crushed connector, water that has bridged the inner and outer conductors, or a pinch point in buried cable.
Impedance increase (higher Z than cable Z₀): Partial positive reflection. The waveform steps up slightly (not as large as an open circuit). This indicates a point where the cable's characteristic impedance increases — a dry or damaged section where the dielectric has degraded, or a connector with poor contact increasing the path impedance.
Impedance decrease (lower Z than cable Z₀): Partial negative reflection. The waveform steps down slightly. Indicates a section where impedance decreases — a water-ingress section where the dielectric's permittivity has increased, reducing the impedance.
Matched termination (50 Ω at far end): No reflection — the waveform is flat after the initial step. This is the TDR calibration check: connect a known 50 Ω termination to the far end of a cable, and the TDR should show no reflection above the cable loss.
TDR waveform signatures: flat = matched 50 Ω termination; positive step = open circuit or impedance increase; negative step = short circuit or impedance decrease. The horizontal position of the step gives the distance to the discontinuity via the TDR cable length formula.
View LargerUsing a VNA as a TDR
A VNA measures S11 as a function of frequency. The time-domain transformation (inverse Fourier transform of the frequency-domain S11 data) converts this frequency-domain measurement into a time-domain response equivalent to a TDR measurement. This capability is available in the NanoVNA companion software (NanoVNA-Saver and nanoVNA-App) and is called the "distance to fault" mode or time-domain reflectometry mode.
To use a VNA as a TDR:
- Set a wide frequency range — the wider the range, the better the time resolution. 10 MHz to 300 MHz provides about 3 ns time resolution (≈ 2 ft at VF = 0.66)
- Calibrate the VNA at the cable input using SOL calibration
- Connect the cable under test to Port 1
- In the software, select the time-domain (TDR) display mode and enter the cable's velocity factor
- Read the distance to each discontinuity from the horizontal axis
The time-domain resolution is limited by the frequency span: time resolution ≈ 1 / (frequency span). A 300 MHz span gives ≈ 3.3 ns time resolution. In a VF = 0.66 cable, this corresponds to about 1.3 feet (0.4 m) of spatial resolution — sufficient for most cable fault location tasks.
Worked Example: Locating a Fault
A 200-foot run of RG-213 (VF = 0.659) from the shack to a tower-mounted antenna shows intermittent high SWR. The transceiver end measures an SWR of 4:1 when the antenna has been verified to be fine (it reads good SWR on a separate cable). TDR measurement procedure:
VNA connected at the shack end of the buried coax. Frequency sweep: 10–400 MHz (span = 390 MHz, time resolution ≈ 2.6 ns ≈ 1 ft). VF entered as 0.659.
The time-domain display shows:
- A small positive step at 53 ns (one-way travel time)
- A larger positive step at 107 ns (the far end of the cable)
Distance to first discontinuity:
d = (t × VF × c) = 53 ns × 0.659 × 0.984 ft/ns = 53 × 0.648 = 34.4 feet
The positive step indicates an impedance increase — likely a water-filled connector or a damaged section where the dielectric has partially dried out, raising the impedance. The fault is at 34 feet from the shack end — likely the first in-line connector along the buried run. Digging at that point and replacing the connector resolves the intermittent SWR problem.
TDR Cable Length Calculator
Enter the round-trip TDR time delay (in nanoseconds) and the cable's velocity factor to calculate the distance to the discontinuity. You can also enter distance and velocity factor to calculate the expected TDR round-trip time — useful for verifying calibration with a known cable length.
TDR Distance / Time Delay Calculator
Enter the round-trip time delay in nanoseconds and the cable velocity factor to find the one-way distance to the discontinuity. Or enter distance and VF to find the expected round-trip time delay.
Frequently Asked Questions
Can a TDR find a fault in a coaxial cable with the antenna still connected?
Yes, but the antenna's impedance (which is not 50 Ω at most frequencies) creates a large reflection at the cable's far end that can mask smaller reflections from faults along the cable. Disconnecting the antenna and terminating the far end with a 50 Ω load gives a cleaner TDR display and makes it easier to see small faults. If the antenna cannot be disconnected, the far-end reflection from the antenna simply appears as a large positive or negative step at the corresponding distance, and you look for additional smaller steps between the source and the antenna — those are the faults in the cable.
How do I measure the velocity factor of an unknown cable using a VNA?
Cut a piece of the unknown cable to a known physical length (e.g., exactly 10 feet). Connect it to the VNA Port 1 with the far end open. Use the VNA in time-domain mode and read the distance displayed for the far-end open reflection. The displayed distance equals the true physical length only if the correct VF is entered. Adjust the VF entry until the displayed distance matches the known physical length. Alternatively, measure S11 in frequency domain with the far end open — the spacing between the S11 minima (anti-resonances) is c × VF / (2 × cable length); rearrange to solve for VF.
What is the minimum fault distance a NanoVNA can resolve?
Time-domain resolution is approximately 1 / (frequency span). The NanoVNA V2 can sweep to 3 GHz, giving a theoretical time resolution of about 0.33 ns, corresponding to about 2.4 inches (6 cm) in RG-213 cable. In practice, the NanoVNA's limited dynamic range and phase noise reduce effective resolution to perhaps 1–2 ns, or about 1 foot (30 cm) in typical cable. This is adequate for locating buried cable faults and identifying connector problems, but insufficient for high-precision laboratory cable characterization.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.