Published on 10/13/2026
An engineer builds a phase-matched pair and cuts both cables to the same length, then measures a 20 degree difference at 6 GHz. One cable was foam dielectric and the other solid PTFE. Same length, different velocity factor, different phase. That is why velocity factor tables matter.
Short answer: velocity factor is the speed of a signal in the cable as a fraction of the speed of light. Solid PE cables such as RG58 and RG213 are about 0.66, PTFE cables such as RG316 and RG402 about 0.70, and foam low-loss cables such as LMR-400 about 0.85.
The table below lists values for common cables, plus the formulas to turn velocity factor into delay and electrical length.
On This Page
| Cable | Dielectric | Velocity factor | Delay (ns/m) |
|---|---|---|---|
| RG58 | Solid PE | 0.66 | 5.05 |
| RG59 | Solid PE | 0.66 | 5.05 |
| RG174 | Solid PE | 0.66 | 5.05 |
| RG213 / RG214 | Solid PE | 0.66 | 5.05 |
| RG223 | Solid PE | 0.66 | 5.05 |
| RG316 / RG316D | PTFE | 0.70 | 4.76 |
| RG178 / RG179 | PTFE | 0.69 to 0.70 | 4.8 |
| RG142 / RG400 | PTFE | 0.69 to 0.70 | 4.8 |
| RG402 / RG405 semi-rigid | PTFE | 0.70 | 4.76 |
| LMR-195 class | Foam PE | 0.80 | 4.17 |
| LMR-240 class | Foam PE | 0.84 | 3.97 |
| LMR-400 class | Foam PE | 0.85 | 3.92 |
| LMR-600 class | Foam PE | 0.87 | 3.83 |
| RG6 (foam) | Foam PE | 0.78 to 0.83 | 4.0 to 4.3 |
| Air-spaced hardline | Air | 0.90 to 0.95 | 3.5 to 3.7 |
Values are typical; check the datasheet because foam levels vary. Delay in ns per meter is 3.336 divided by the velocity factor.
Key idea: A cable is electrically longer than its physical length. RG58 behaves like 1.5 times its length, and RG316 like 1.43 times.
Phase-matched assemblies
If your design depends on delay or phase, we can build and sweep matched pairs on a VNA. Tell us the frequency and tolerance.
Talk to an Engineer| Application | Why it matters |
|---|---|
| Phase-matched arrays and MIMO | Length mismatch becomes phase error |
| Stubs, baluns and matching sections | Cut length must use VF, not free space |
| Timing and GPS cable delay | Delay is VF dependent |
| TDR fault location | Distance to a fault needs the right VF setting |
| Antenna phasing lines | Quarter wave sections need exact VF |
For wavelength numbers, see the frequency to wavelength table in this series, and the cable length in wavelengths guide on this blog.
Avoid
What is the velocity factor of RG58?
About 0.66.
What is the velocity factor of RG316?
About 0.70, due to its PTFE dielectric.
What is the velocity factor of LMR-400?
About 0.85.
How do I calculate delay from velocity factor?
Delay in ns per meter is 3.336 divided by the velocity factor.
Does velocity factor change with frequency?
Slightly. It varies by a fraction of a percent across the usable band.
Why do foam cables have higher velocity factor?
Foam contains air, which lowers the effective dielectric constant and raises speed.
Use the table to convert physical length to electrical length and delay. For phase-critical work, measure the actual cable on a VNA rather than relying on the nominal figure.
SigmaRF builds phase-matched and standard assemblies in India (Vadodara) and the USA (Cleveland area), VNA-swept before shipment, with lead times as short as 2 days.
Order matched assemblies
Send the frequency and phase tolerance.
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