Calculator Inputs
Use the direct factor when a datasheet gives VF. Use dielectric mode when you only know relative permittivity.
Example Data Table
These example values are typical references only. Always confirm actual datasheet values before using them for production timing, matching, or RF design.
| Cable Type | Nominal Impedance | Dielectric | Typical Velocity Factor | Typical εr | Notes |
|---|---|---|---|---|---|
| RG-58 | 50 Ω | Solid polyethylene | 0.66 | 2.25 | Common lab and radio cable. |
| RG-6 | 75 Ω | Foam polyethylene | 0.82 | 1.49 | Often used for video and broadband. |
| LMR-400 | 50 Ω | Foam dielectric | 0.85 | 1.38 | Low-loss feedline example. |
| CAT6 UTP | 100 Ω | Twisted pair insulation | 0.69 | 2.10 | NVP varies by manufacturer. |
| PTFE coax | 50 Ω | PTFE | 0.70 | 2.04 | Stable across wide temperatures. |
Formula Used
VF = 1 / √εr
v = c × VF
where c = 299,792,458 m/s
Delay = Cable Length / Propagation Speed
RTD = 2 × Cable Length / Propagation Speed
λ0 = c / f
λcable = v / f = (c × VF) / f
Phase Shift = 360 × Cable Length / λcable
How to Use This Calculator
- Enter a cable label for your own reference.
- Enter the physical cable length and select its unit.
- Enter the operating frequency and choose the correct frequency unit.
- Select direct velocity factor when your datasheet already gives VF.
- Select dielectric constant when you only know εr.
- Add an optional tolerance percentage to see likely timing spread.
- Press Calculate to show results above the form.
- Use the CSV or PDF buttons to export the visible result summary.
Frequently Asked Questions
1) What is cable velocity factor?
Cable velocity factor is the ratio of signal speed in a cable to the speed of light in free space. A value of 0.66 means signals travel at 66% of light speed inside that cable.
2) How does dielectric constant affect velocity factor?
A higher dielectric constant slows the signal more. For many cable models, velocity factor is approximated by 1 divided by the square root of the relative permittivity.
3) Why is signal speed lower inside a cable?
The insulating material stores electric energy and changes field propagation. That dielectric loading reduces wave speed compared with free-space propagation.
4) Should I use datasheet VF or calculated VF?
Use the datasheet value whenever it is available. Manufacturer data usually reflects the real construction, geometry, and material mix better than a simple dielectric-only approximation.
5) Does cable length change the velocity factor?
No. Velocity factor is mainly a material and construction property. Cable length changes total delay and phase shift, but it does not change the underlying propagation ratio itself.
6) Why is wavelength shorter inside the cable?
Wavelength equals speed divided by frequency. Frequency stays the same, but signal speed drops inside the cable, so the wavelength becomes shorter in the dielectric medium.
7) What tolerance value is reasonable?
Use the cable datasheet if it gives a nominal velocity of propagation tolerance. If no tolerance is listed, a small engineering estimate such as 1% to 5% is often used for preliminary analysis.
8) When is round-trip delay important?
Round-trip delay matters in reflection studies, TDR interpretation, timing loops, echo paths, and protocols where the signal must travel to the far end and back.