Coax Electrical Length Calculator
Work out the physical length of coax to cut for a given electrical length, accounting for the cable's velocity factor. Enter a frequency, the velocity factor of your cable and the electrical length in wavelengths (0.25 for a quarter-wave stub) to get the length in metres and centimetres, plus the free-space wavelength.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the operating frequency f in MHz.
- Enter the cable's velocity factor VF (default 0.66 for solid-PE coax like RG-58/RG-213; use ~0.8 for foam dielectric — check the datasheet).
- Enter the electrical length in wavelengths (default 0.25 = quarter-wave; use 0.5 for a half-wave, etc.) and read off the physical cut length.
How it works
The free-space wavelength is λ = 300 / f, with f in MHz and λ in metres (this uses c ≈ 3×10⁸ m/s). Because the signal travels slower inside the cable, the physical length for a fraction of a wavelength is length = fraction × VF × λ, where VF is the velocity factor. For example a quarter-wave (0.25 λ) at 100 MHz on VF 0.66 cable is 0.25 × 0.66 × 3 = 0.495 m.
Worked example
Worked example. At f = 100 MHz with VF = 0.66 and a quarter-wave (0.25 λ): λ = 300 / 100 = 3 m, so the cut length = 0.25 × 0.66 × 3 = 0.495 m = 49.5 cm.
Common mistakes
- Using the free-space length and forgetting the velocity factor — the cut coax is always shorter than the free-space wavelength.
- Assuming every cable has VF 0.66 — foam and air-spaced cables can be 0.8–0.9, giving a noticeably longer cut.
- Cutting exactly to the calculated length and soldering — always cut long and trim to resonance, since real VF varies from the nominal figure.
Frequently asked questions
What is a velocity factor?
It is the ratio of the speed of a signal in the cable to the speed of light in free space. Solid polyethylene coax is around 0.66, foam dielectric around 0.8 and air-spaced/hardline can exceed 0.85.
Why is the cut coax shorter than a free-space quarter-wave?
The signal moves slower in the dielectric, so a given electrical length (a fraction of a wavelength) fits into a shorter physical length — exactly the VF factor shorter.
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Tip: Enter any known values to calculate the remaining results.
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