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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.

VNAopenL 5 mfirst resonance 9.9 MHz
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
First resonance frequency (MHz)
Spacing between resonances (MHz), if measured
Cable characteristic impedance (Ω)
Type to compare against
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. «Cable velocity factor calculator». 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.

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