Cable velocity factor calculator

Two minutes and no guesswork

You need the velocity factor for any section cut to a calculated length — a transformer, a stub, a phasing line. Taking it from a table is risky: a drum bought at the market often carries the wrong type name. Measuring it with an analyser is quicker than hunting for a datasheet.

The first extremum of |Z| falls at a quarter wave in the cable — and that is what vf is worked out from

Physical section length (m)
Far end	Open — looking for the minimum of |Z|Shorted — looking for the maximum of |Z|
First resonance frequency (MHz)
Spacing between resonances (MHz), if measured
Cable characteristic impedance (Ω)
Type to compare against	Enter by handRG-58 (vf 0.66)RG-213/U (vf 0.66)RG-174 (vf 0.66)RG-316 PTFE (vf 0.695)RG-142 PTFE (vf 0.695)RG-8X foam (vf 0.82)RG-6 foam (vf 0.83)H-1000 (vf 0.83)LMR-400 (vf 0.85)Heliax 7/8" (vf 0.88)Open-wire line (vf 0.95)
Result
Velocity factor vf
Datasheet value
Expected spacing between resonances
Permittivity of the insulation
Electrical length
Capacitance per metre

📐 The formulas

From the first resonance:

vf = 4 × L × f₁ / c

On an open-ended section the first minimum of |Z| falls at λ/4; on a shorted one, the first maximum. The formula is the same.

A check using the resonance spacing:

Δf = c · vf / (2L)     →     vf = 2 · L · Δf / c

The two figures should agree. If they do not, the wrong extremum was taken as the first resonance.

Derived quantities:

ε = 1 / vf²
C_per_metre = 1 / (Z₀ · vf · c)      [F/m]
Electrical length = L / vf

Where this calculator stops:

• Measure the length including the connector, or subtract its electrical length — on VHF it already matters
• The first resonance is easy to confuse with the second: always check the result against the resonance spacing
• Deriving ε from vf holds for TEM lines: coax and two-wire yes, waveguide no
• Capacitance per metre is worked out from the rated Z₀; if that is unknown, this figure will be wrong

🎯 In practice

• Before cutting a quarter-wave transformer — so as not to be 5 % out.
• Checking cable bought at a market. A measured vf that does not match the catalogue is the quickest sign of a counterfeit.
• Finding the vf of unknown cable from an old stock.

Practical advice:

1. Use a 3–10 m length: shorter puts the resonance too high, longer packs the resonances too closely.
2. An open end is more convenient: nothing to solder.
3. Measure on the very cable you will cut the section from — even within one type there is spread.
4. Feed the measured vf into the transformer and stub calculators instead of the table value.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator

UR3PKI. «Velocity Factor Calculator: Measure Coax with a NanoVNA». CyberDev.Space. https://cyberdev.space/en/radio/calculators/lines/velocity_factor/ (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.