Cable Length to Electrical Delay Calculator
Created by: Daniel Hayes
Last updated:
Find the propagation delay, round-trip TDR time, and phase shift of any coaxial cable run — useful for phased antenna arrays, fault-finding, and timing-sensitive digital mode setups.
Cable Length to Electrical Delay Calculator
Amateur RadioCalculate one-way and round-trip propagation delay, electrical length in degrees, and phase shift for any coaxial cable run.
What is a Cable Length to Electrical Delay Calculator?
A cable delay calculator converts the physical length of a coaxial cable into the time it takes an RF signal to travel that distance, taking the velocity factor of the cable dielectric into account. Unlike the speed of light in free space, signals in coaxial cable travel at a fraction of that speed — typically 66 to 95 percent depending on the dielectric material. Knowing the exact propagation delay is important for TDR fault-finding, phased antenna arrays, and timing-sensitive digital mode applications.
Amateur radio operators encounter cable delay most often in two practical contexts. The first is building phased antenna arrays for VHF and UHF, where each feed cable must deliver a signal at a specific phase angle relative to the other elements. A 90-degree phasing cable between two elements requires a specific physical length calculated from the velocity factor, not just any quarter-wave guess. Getting this wrong can reduce gain or push the radiation pattern in the wrong direction.
The second context is TDR (time-domain reflectometry), where a pulse or sweep is sent into a cable and the reflected signal from a fault is timed. Converting round-trip delay to fault distance requires the velocity factor. A 100-nanosecond round-trip delay in RG-58 (VF 0.66) corresponds to a fault at about 32 feet from the measurement point, while the same delay in hardline (VF 0.95) would indicate a fault at about 46 feet.
The calculator handles the most common case — a simple uniform-velocity cable — and provides both one-way and round-trip delay along with the electrical phase shift at any frequency. It also shows how the same physical cable length produces different delay values in cables with different velocity factors, reinforcing why knowing your cable type before cutting phasing sections is essential.
How the Cable Length to Electrical Delay Calculator Works
The propagation delay formula is straightforward: delay (ns) = length (ft) / (VF × 983.571 ft/μs) × 1000. The 983.571 ft/μs constant is the speed of light in feet per microsecond. Multiplying by VF slows this to the actual propagation speed in the cable, and multiplying by 1000 converts microseconds to nanoseconds. The electrical phase shift at a given frequency is then delay_μs × frequency_MHz × 360 degrees.
The comparison table shows delay and phase shift for each common cable type at the entered physical length and frequency. This makes it immediately visible that two cables of the same physical length — say 50 feet of RG-58 and 50 feet of LMR-400 — deliver signals with different phase angles at the same frequency. The LMR-400 (VF 0.85) has less phase shift than the RG-58 (VF 0.66) because the signal travels faster through it.
Cable delay formulas
Propagation speed in cable = c × VF where c = 983.571 ft/μs
One-way delay (ns) = length_ft / (983.571 × VF) × 1000
Round-trip delay (ns) = 2 × one-way delay (used for TDR fault distance)
Electrical length (°) = one-way delay_μs × frequency_MHz × 360
TDR fault distance (ft) = round_trip_delay_ns / 2 × (983.571 × VF) / 1000
Phase shift per foot (°/ft) = 360 × frequency_MHz / (983.571 × VF)
Example Calculations
Example 1: TDR fault location in RG-213
A fault reflects back with a 120 ns round-trip delay in RG-213 (VF 0.66). Fault distance = 120/2 × (983.571 × 0.66) / 1000 = 60 × 0.649 = 38.9 feet from the measurement point. This is a common calculation for locating damaged underground cable runs or finding a broken connector inside a wall.
Example 2: Phasing harness for a 2m vertical array
For a 90-degree phasing cable at 144 MHz in LMR-400 (VF 0.85), the required one-way delay is 90 / (144 × 360) × 1,000,000 ns = 1736 ns. Physical length = 1736 × 983.571 × 0.85 / 1,000,000 = 1.45 ft = 17.4 inches. The delay calculator confirms this and also shows the same phase shift for each cable type.
Example 3: Phase shift of a 100-foot VHF feedline
A 100-foot run of RG-58 (VF 0.66) at 144 MHz introduces 100 / (983.571 × 0.66) × 1000 = 154 ns of one-way delay, which corresponds to 154 × 144 × 360 / 1,000,000 = 7,974 degrees — or about 22.2 full wavelengths and 54 degrees of phase shift at the antenna end. This is why feedline length matters when phasing two elements at VHF.
Common Amateur Radio Uses
- Calculate TDR fault distance from measured round-trip delay and known cable velocity factor.
- Design phasing harnesses for stacked antenna arrays or Yagi phased-feed systems where electrical length controls beam direction.
- Verify that two feed cables are matched in electrical length (phase delay) before connecting to a phased array.
- Understand timing relationships in digital mode transceiver setups where cable delays are rarely the limiting factor but are occasionally worth checking.
- Convert a measured resonant frequency of a coax stub back to cable velocity factor for unknown or unmarked cable identification.
- Plan cable routing for remote station installations where knowing the electrical length of long feedline runs helps with antenna tuner and phasing design.
Tips for Better Ham Radio Planning
For phased array applications, always measure the final assembly with a VNA rather than relying purely on calculated cut lengths. Cable tolerances, connectors, and the antenna element impedance all interact with the phasing network. Trim cables to resonance with the array connected and monitor the radiation pattern rather than just the calculated delay. Even 5 to 10 degrees of phase error can noticeably affect a two-element cardioid pattern.
For TDR measurements, a wider bandwidth pulse gives better range resolution. Antenna analysers like the RigExpert AA-600 or NanoVNA can perform basic TDR sweeps using an inverse Fourier transform of the impedance versus frequency data. The speed of light constant and velocity factor are built into the instrument, but entering the correct VF for your cable type is still essential for accurate fault distance readings.
Frequently Asked Questions
What is cable propagation delay and why does it matter in ham radio?
Cable propagation delay is the time it takes an RF signal to travel the length of a coaxial cable. At 1 nanosecond per foot (in free space), a 50-foot cable adds 50 ns of one-way delay, or 100 ns round trip. This matters in TDR fault-finding, phased antenna arrays where each element must be fed at a precise phase angle, and timing-sensitive digital mode decoders where cable delay can offset transmit and receive windows.
How does velocity factor affect propagation delay?
Velocity factor (VF) sets how fast the signal travels through the cable relative to free space. A cable with VF 0.66 carries signals at 66 percent of light speed, so it takes 1/0.66 = 1.52 ns per foot instead of the 1.02 ns per foot of free-space travel. Two cables of the same physical length but different VF will deliver signals at different times and phase angles — a critical difference when building phasing arrays or testing with time-domain reflectometry.
What is time-domain reflectometry and how does cable delay help?
Time-domain reflectometry (TDR) is a technique for finding faults and cable breaks by sending a fast pulse into one end and measuring the round-trip travel time of the reflected pulse from the fault. Knowing the velocity factor of the cable allows you to convert round-trip delay directly to fault distance: distance = (round_trip_delay_ns / 2) × (VF × 983.6 ft/μs) / 1000. Many antenna analysers and SDRs can perform TDR-style measurements for finding damaged coax.
How much phase shift does 50 feet of RG-58 add at 2 metres?
At 144 MHz with VF 0.66, 50 feet of RG-58 introduces about 353 degrees of phase shift — nearly a full wavelength. This means the signal is almost back to its starting phase, which is why the exact feedline length to a phased array element must be controlled to within a fraction of a wavelength rather than just any convenient length.
Can I use delay values to check if two feedlines are matched in length?
Yes. For antenna stacking harnesses and phased arrays, you need equal electrical lengths (equal phase delay) to each element, not necessarily equal physical lengths. Two cables of different types with equal calculated delay at your operating frequency will deliver signals in phase. Measure the one-way delay with a VNA or antenna analyser port-to-port measurement, or calculate it from physical length and published VF.
Why is round-trip delay important for digital modes?
Digital modes like WSPR, FT8, and JS8Call rely on accurate timing synchronisation between transmitting and receiving stations. The one-way cable delay from the radio to the antenna is typically under 0.1 microseconds even on a 100-foot run — far too small to affect GPS-disciplined timing. However, the total system latency from audio interface, DSP, and USB can matter, and for VHF weak-signal digital modes with tight timing windows, every source of uncertainty is worth understanding.
Sources and References
- ARRL Antenna Book, 24th edition — Transmission Lines chapter, propagation delay and TDR applications.
- ARRL Handbook for Radio Communications — Time-domain reflectometry techniques for amateur radio.
- Times Microwave LMR technical specifications — velocity factor and propagation delay data.
- Pozar, D.M., Microwave Engineering, 4th edition — Transmission line propagation and phase velocity.