Coax Velocity Factor Calculator
Calculate coaxial cable velocity factor, propagation velocity, propagation delay, and wavelength scaling from the dielectric constant (relative permittivity).
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Math
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Related
Enter parameters and click Calculate to view results
Formula & Theory
VF = 1 / √εr, Velocity = c × VF, Delay = 1 / (c × VF), λc = λ × VFThis formula is used to calculate antenna parameters for coax velocity factor calculator.
Overview
The Coax Velocity Factor Calculator computes the Velocity Factor ($VF$), signal propagation speed, unit delay (ns/m), and guided wavelength scaling for coaxial transmission lines based on the insulating material's relative dielectric permittivity ($epsilon_r$). It helps RF engineers and antenna builders accurately dimension transmission lines, stub tuners, and phasing harnesses.
Input Guide
Enter Dielectric Constant (εr) exactly in the units shown by this coax velocity factor. Check the operating band, unit prefix, and decimal position before calculating; these are the inputs used by the formula.
- Dielectric Constant (εr).
Output Guide
The results describe the calculated coax velocity factor values for the inputs you entered. Check each value against the available space, selected components, feed system, and operating conditions before making a final design decision.
How This Calculator Works
The Coax Velocity Factor uses VF = 1 / √εr, Velocity = c × VF, Delay = 1 / (c × VF), λc = λ × VF. Supply Dielectric Constant (εr) in the displayed units, then use the calculated values as the first engineering target for this transmission lines design or analysis.
Design Notes
When radio frequency energy propagates down a coaxial line, the surrounding dielectric insulator slows the wave velocity relative to vacuum speed ($c$). The relationship is defined by $VF = 1 / sqrt{epsilon_r}$. For example, solid polyethylene ($epsilon_r \approx 2.25$) yields $VF = 0.66$ ($66\%$ of $c$), foam polyethylene ($epsilon_r \approx 1.44$) gives $VF = 0.83$, and PTFE ($epsilon_r \approx 2.07$) yields $VF = 0.69$. A lower velocity factor shortens guided wavelength ($lambda_c = lambda_0 cdot VF$), requiring physical stub lengths to be trimmed accordingly.
Build and Tuning Notes
Because dielectric manufacturing tolerances can vary by $pm2\% ext{ to }pm5\%$, measure the precise velocity factor of a cable spool using a Vector Network Analyzer (VNA) or Time-Domain Reflectometer (TDR) before cutting critical impedance-matching quarter-wave stubs or phase-matched phasing lines. Temperature variations also induce slight changes in dielectric permittivity and delay.
Frequently Asked Questions
How is Velocity Factor calculated from the Dielectric Constant?
Velocity Factor ($VF$) is inversely proportional to the square root of the relative dielectric constant ($epsilon_r$): $VF = 1 / sqrt{epsilon_r}$.
Why does Velocity Factor matter when cutting resonant stubs?
Because signals travel slower inside a cable, the wavelength inside the dielectric is shorter than in free space ($lambda_c = lambda_0 cdot VF$). Physical quarter-wave stubs must be multiplied by $VF$ to match electrical resonance.
What are typical Velocity Factors for common coaxial cables?
Solid PE dielectric (RG-58, RG-213) is $\approx 0.66$, PTFE/Teflon (RG-142, RG-316) is $\approx 0.69\text{--}0.70$, Foam PE (LMR-400, RG-8X Foam) is $\approx 0.80\text{--}0.85$, and Air-articulated/Heliax is $\approx 0.88\text{--}0.93$.
How does Velocity Factor affect propagation delay?
Propagation delay increases as Velocity Factor drops. The delay per meter is $\tau = 3.335 / VF\text{ ns/m}$. A $66\%$ $VF$ cable introduces $\approx 5.06\text{ ns/m}$ of delay, whereas an $85\%$ $VF$ cable introduces $\approx 3.92\text{ ns/m}$.
Alex Warren
B.Sc. in Electrical & Electronic Engineering (EEE)
Alex specialises in antenna design and wave propagation. His expertise helps ensure these calculators present practical RF concepts, useful design estimates, and clear engineering guidance for students, HAM operators, and wireless professionals.