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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Enter parameters and click Calculate to view results
Formula & Theory
VF = 1 / √epsilon r, Velocity = c × VF, Delay = 1 / (c × VF), lambdac = lambda × 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 (epsilon r) exactly as shown on the calculator. Confirm every unit, selected option, and decimal position before calculating so the coax velocity factor result matches the intended transmission lines design case.
Output Guide
The output section reports Velocity Factor, Propagation Velocity, Propagation Speed, Delay per Metre, Delay per 100 m, Wavelength Scaling, Typical Cable Type for the values you entered. Use these values as design targets, then compare them with available space, component limits, feed system behavior, installation environment, and measured performance before finalizing the design.
How This Calculator Works
The Coax Velocity Factor applies VF = 1 / √epsilon r, Velocity = c × VF, Delay = 1 / (c × VF), lambdac = lambda × VF to the entered values. Calculate coaxial cable velocity factor, propagation velocity, propagation delay, and wavelength scaling from the dielectric constant (relative permittivity). Use the result as a first-pass transmission lines target, then validate it against losses, tolerances, mounting, nearby conductors, feed-line effects, and measurement conditions.
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 approximately 2.25) yields VF = 0.66 (66% of c), foam polyethylene (epsilon_r approximately 1.44) gives VF = 0.83, and PTFE (epsilon_r approximately 2.07) yields VF = 0.69. A lower velocity factor shortens guided wavelength (lambda_c = lambda_0 * VF), requiring physical stub lengths to be trimmed accordingly.
Build and Tuning Notes
Because dielectric manufacturing tolerances can vary by +/-2% to +/-5%, 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.
Inputs used by this calculator
- Dielectric Constant (epsilon r).
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 * 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 approximately 0.66, PTFE/Teflon (RG-142, RG-316) is approximately 0.69--0.70, Foam PE (LMR-400, RG-8X Foam) is approximately 0.80--0.85, and Air-articulated/Heliax is approximately 0.88--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 ns/m. A 66% VF cable introduces approximately 5.06 ns/m of delay, whereas an 85% VF cable introduces approximately 3.92 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.