Coax Velocity Factor Calculator
Created by: Liam Turner
Last updated:
Find the exact physical cut length for quarter-wave stubs, phasing cables, and matching sections by applying the velocity factor of your specific coax type.
Coax Velocity Factor Calculator
Amateur RadioConvert between physical and electrical cable lengths, find stub cut dimensions, and compare velocity factors across common cable types.
What is a Coax Velocity Factor Calculator?
A coax velocity factor calculator converts between the physical length of a cable and its electrical length in degrees or wavelength fractions. Velocity factor (VF) is the ratio of signal propagation speed inside a coaxial cable to the speed of light in free space. Because RF signals travel slower inside real cables than in air, a given physical length corresponds to a longer electrical wavelength — and cutting a stub or matching section to the right physical dimension requires knowing the exact velocity factor for the cable type being used.
This matters most when constructing quarter-wave stubs, half-wave phasing sections, or impedance transformers. A quarter-wave section at 144 MHz in free space is about 20.5 inches, but in RG-58 with VF 0.66, the physical quarter-wave is only about 13.5 inches. Cutting to the free-space number produces a section that is too long — resonating at a lower frequency than intended — which can worsen SWR rather than improve it.
Velocity factor varies significantly between cable types. Solid polyethylene dielectric cables like RG-58 and RG-213 have VF near 0.66 because solid PE has a permittivity of about 2.3. Foam dielectric cables like LMR-400 use a partially air-filled foam PE, raising VF to 0.83 to 0.87. Air-spaced hardline coax can reach VF of 0.95 to 0.97. Open-wire feeders and ladder line run even higher because most of the field is in air.
Amateur radio operators encounter velocity factor most often when building VHF and UHF antenna systems. Antenna stacking harnesses, phasing cables for two-antenna arrays, and gamma-match or beta-match assemblies all depend on precise physical cut lengths derived from the velocity factor. The calculator also covers HF applications like resonant coax traps and loading coil bypass stubs.
How the Coax Velocity Factor Calculator Works
The core formula is straightforward: the physical length of a cable for a given electrical angle is (angle / 360) × (c × VF / f), where c is the speed of light in the same units as the desired length, VF is the velocity factor, and f is frequency. For finding electrical length from a known physical cable, the formula is inverted: angle = (physical_length / (c × VF / f)) × 360. The calculator applies these formulas and also computes the one-way propagation delay in nanoseconds.
The comparison chart shows the physical length required for a quarter-wave (90°) section at the entered frequency for each of the common cable types. This makes it immediately clear why low-VF cables like RG-58 need shorter physical stubs than high-VF cables like ladder line — and why measuring with an antenna analyser after cutting is always recommended for precision VHF/UHF work.
Velocity factor formulas
Velocity factor: VF = v_phase / c (ratio, 0 to 1)
Wavelength in cable: λ_cable (ft) = 983.571 × VF / f(MHz)
Physical length for N degrees: L_physical (ft) = (N / 360) × λ_cable
Electrical length of physical length: N (°) = (L_physical / λ_cable) × 360
Propagation delay: delay (ns) = L_physical_ft / (983.571 × VF) × 1000
Quarter-wave physical: L_λ/4 (ft) = 245.89 × VF / f(MHz)
Example Calculations
Example 1: Quarter-wave RG-58 stub at 144 MHz
At 144 MHz with VF 0.66 (RG-58), the physical quarter-wave section is 245.89 × 0.66 / 144 = 1.127 ft or 13.5 inches. This is the cut length for a short-circuit stub that produces an open circuit (very high impedance) in parallel at 144 MHz, or an open stub that produces a short circuit — useful for band-rejection filters or gamma-match assemblies.
Example 2: Phasing harness for a 2m vertical array
A 90-degree phasing cable for a two-element array at 146 MHz in LMR-400 (VF 0.85) is 245.89 × 0.85 / 146 = 1.431 ft = 17.2 inches. This precise cut length is critical for achieving the proper phase relationship between elements. Using RG-8X (VF 0.82) would give 245.89 × 0.82 / 146 = 1.381 ft — nearly 0.5 inches shorter.
Example 3: Half-wave coax section for remote impedance check
A half-wave section of coax presents the antenna feed-point impedance at the shack end of the feedline, regardless of Z₀. For 40 metres (7.15 MHz) in RG-213 (VF 0.66), the half-wave physical length is 491.79 × 0.66 / 7.15 = 45.4 feet. Measuring impedance through this section tells you exactly what the antenna presents at that frequency.
Common Amateur Radio Uses
- Cut stub sections for VHF and UHF matching, band rejection, or gamma-match assemblies to the correct physical length for the cable being used.
- Build phasing harnesses for antenna arrays where each section must deliver a specific electrical phase delay between elements.
- Determine the physical half-wave or quarter-wave lengths for coaxial traps used to achieve multiband operation on wire antennas.
- Understand why two cables of the same physical length but different dielectric types have different electrical properties at the same frequency.
- Calibrate or verify the velocity factor of a cable sample using a known frequency and measured resonant length.
- Plan portable antenna builds for POTA and SOTA where keeping stubs and matching sections compact and lightweight matters.
Tips for Better Ham Radio Planning
Always cut stubs slightly long on the first pass and trim to the exact resonant length using an antenna analyser or VNA. Cable manufacturing tolerances mean the actual VF can differ slightly from the datasheet value, and connectors add a small additional electrical length. Trim in small increments from the open end and check SWR or impedance after each cut. Once the section is too short, there is no easy fix.
Temperature affects velocity factor by a small amount as the dielectric expands and contracts. For most amateur applications this is negligible, but for high-precision repeater duplexer cavities and professional installations, VF can shift by 0.001 to 0.002 over a 30°C temperature range. When building duplexer stubs intended for outdoor use, plan for final field-trimming rather than relying entirely on bench measurements.
Frequently Asked Questions
What is velocity factor in coaxial cable?
Velocity factor (VF) is the ratio of the signal propagation speed inside the cable to the speed of light in free space. A cable with VF 0.66 carries signals at 66 percent of light speed. This means a physical metre of that cable is electrically equivalent to 1/0.66 = 1.52 metres of free-space transmission. VF is determined by the permittivity of the dielectric material surrounding the centre conductor.
Why does velocity factor matter for cutting cable stubs?
When you cut a quarter-wave stub or matching section, you must use the physical length for the cable you are using, not the free-space quarter-wave. A quarter-wave at 144 MHz in free space is about 20.5 inches, but in RG-58 (VF 0.66) the physical quarter-wave is only about 13.5 inches. Ignoring velocity factor results in a stub that resonates at the wrong frequency, often making SWR worse rather than better.
How do I measure the velocity factor of an unknown cable?
The most practical method is to cut a known physical length of cable, then use a noise bridge, antenna analyser, or VNA to find the frequency at which the cable is a quarter-wave (open-circuit input shows minimum impedance, or short-circuit input shows maximum impedance). VF = (physical length / free-space quarter-wave at that frequency). The free-space quarter-wave at f MHz is 246/f feet.
Do coaxial connectors affect the electrical length?
Yes, connectors add a small amount of additional electrical length that is not captured in the VF of the cable itself. For HF applications this is negligible, but at VHF and UHF — particularly for phasing harnesses and precise stub cutting — connector electrical lengths can shift stub resonance by several degrees. High-quality calibration techniques or a VNA allow you to trim a stub to its target electrical length after assembly rather than relying purely on calculated cuts.
What is the electrical length of a half-wave coax section?
A half-wave coax section has the property of presenting the load impedance at its input regardless of the characteristic impedance of the cable. This is useful for remote impedance measurement, for running feedline any convenient multiple of half-waves without altering the impedance at the transmitter, or for constructing hairpin stubs. The physical half-wave for your cable is 492/f × VF in feet, where f is in MHz.
What is the velocity factor of ladder line and why is it different from coax?
Ladder line and twin-lead have higher velocity factors (0.82–0.95) than solid-dielectric coax because most of the electric field travels in the air between the two conductors rather than through a dense dielectric. Air has a permittivity very close to 1.0, which corresponds to VF = 1.0. The plastic spacers holding the conductors apart lower the effective permittivity slightly, resulting in VF slightly below 1.0 rather than the 0.66 typical of solid-PE coax.
Sources and References
- ARRL Antenna Book, 24th edition — Transmission Lines chapter, velocity factor and stub design tables.
- ARRL Handbook for Radio Communications — Coaxial cable specifications and velocity factor data.
- Times Microwave LMR technical datasheets — velocity factor specifications for LMR-240, LMR-400, and LMR-600.
- Belden technical data — RG-58 and RG-213 velocity factor and electrical specifications.