Cable Length Calculator (TDR)

Time Domain Reflectometry (TDR) is a technique used to measure cable length by sending a signal pulse down a cable and timing how long it takes to bounce back. Enter your velocity factor, round trip time (in nanoseconds), cable type, and preferred unit system into the Cable Length Calculator (TDR) to find the cable length in meters or feet. Secondary outputs include one-way travel time and signal speed inside the cable.

Velocity of propagation factor for the transmission line (typically 0.66 for coax)

ns

Time for signal to travel to end of cable and back in nanoseconds

Results

Cable Length

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Cable Length (Feet)

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One-Way Travel Time

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Signal Speed in Cable

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Frequently Asked Questions

What is Time Domain Reflectometry (TDR) technology?

TDR technology sends electrical pulses down a cable and measures the time it takes for reflections to return. By analyzing these reflections and knowing the cable's velocity factor, it can accurately determine cable length and locate faults.

What is velocity factor and why is it important?

Velocity factor (VF) is the ratio of signal propagation speed in the cable compared to the speed of light in vacuum. Different cable types have different VF values - coax typically has 0.66, while data cables are around 0.64. Using the correct VF is crucial for accurate length measurements.

How accurate are TDR cable length measurements?

TDR measurements are typically accurate within ±2% plus 3 feet for most cable types. Accuracy depends on using the correct velocity factor for your specific cable type and proper calibration of the TDR instrument.

What types of cables can be measured with TDR?

TDR can measure most electrical cables including coaxial cables, twisted pair data cables (CAT5/CAT6), Romex electrical cables, twin lead cables, and many other wire types. Each cable type requires its specific velocity factor for accurate results.

Can TDR detect cable faults and breaks?

Yes, TDR technology can locate opens, shorts, and impedance mismatches in cables. The reflection characteristics change at fault locations, allowing technicians to pinpoint the exact distance to the problem area.

What is round-trip time in TDR measurements?

Round-trip time is the total time for a signal to travel from the TDR instrument to the end of the cable and back. This time, combined with the velocity factor, is used to calculate the actual cable length.

How do I choose the right velocity factor for my cable?

Check your cable specifications or use standard values: Coax (0.66), CAT5/CAT6 (0.64), Romex (0.62), Twin Lead (0.82). For unknown cables, you can measure a known length sample to determine the correct velocity factor.

What are the advantages of TDR over traditional measurement methods?

TDR provides non-destructive testing, can measure installed cables without access to both ends, locates faults precisely, works on energized or de-energized circuits, and offers high accuracy for both length measurement and fault location.