Coaxial Cable Impedance Calculator
Calculate characteristic impedance, velocity factor, propagation velocity, and other transmission line parameters of a coaxial cable.
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Inputs
Live
Math
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Related
Enter parameters and click Calculate to view results
Formula & Theory
Z₀ = (60 / √epsilon r) × ln(D/d)This formula is used to calculate antenna parameters for coaxial cable impedance calculator.
Overview
The Coaxial Cable Impedance Calculator determines the characteristic impedance (Z_0), distributed capacitance (pF/m), velocity factor (VF), and signal delay for coaxial transmission lines. By entering the inner shield diameter (D), center conductor diameter (d), and insulator dielectric constant (epsilon_r), RF engineers and cable designers can quickly evaluate custom coaxial geometries.
Input Guide
Enter Inner Shield Diameter exactly as shown on the calculator. Confirm every unit, selected option, and decimal position before calculating so the coaxial cable impedance result matches the intended waveguide & feedline design case.
Output Guide
The output section reports Characteristic Impedance, Diameter Ratio (D/d), Velocity Factor, Propagation Velocity, Signal Delay, Estimated Capacitance, Recommended 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 Coaxial Cable Impedance applies Z₀ = (60 / √epsilon r) × ln(D/d) to the entered values. Calculate characteristic impedance, velocity factor, propagation velocity, and other transmission line parameters of a coaxial cable. Use the result as a first-pass waveguide & feedline target, then validate it against losses, tolerances, mounting, nearby conductors, feed-line effects, and measurement conditions.
Design Notes
Coaxial cable characteristic impedance is determined solely by physical dimensions and insulator dielectric permittivity: Z_0 = 60 / sqrt(epsilon_r) ln(D / d) ohms. Standard 50 ohms coax represents an optimal trade-off between maximum power handling capability (which peaks near 30 ohms) and minimum voltage breakdown/attenuation (which peaks near 77 ohms). Television and broadband CATV distribution networks utilize 75 ohms coax because it minimizes signal attenuation per unit length.
Build and Tuning Notes
When manufacturing or selecting custom coaxial cables, ensure concentric alignment of the center conductor within the shield dielectric—eccentricity introduces unwanted higher-order modes and lowers voltage breakdown ratings. Validate characteristic impedance and return loss (S_11) across operating frequencies using a Vector Network Analyzer (VNA) or Time-Domain Reflectometer (TDR).
Inputs used by this calculator
- Inner Shield Diameter — use mm.
- Centre Conductor Diameter — use mm.
- Dielectric Constant (epsilon r).
Frequently Asked Questions
Why is 50 ohms the standard impedance for RF and amateur radio?
For an air-dielectric coaxial line, minimum attenuation occurs at 77 ohms, while maximum RF power handling occurs at 30 ohms. 50 ohms was chosen as an ideal commercial compromise between high power handling and low signal attenuation.
Why is 75 ohms used for TV and video signals?
Because video receiver systems prioritize low signal attenuation over high RF power handling, 75 ohms is chosen for CATV, satellite downlinks, and video distribution to minimize signal loss over long cable runs.
How does dielectric permittivity (epsilon r) affect characteristic impedance?
Increasing dielectric constant (epsilon_r) reduces characteristic impedance (Z_0 is proportional to 1/sqrt(epsilon_r)) while simultaneously decreasing propagation velocity (VF = 1/sqrt(epsilon_r)) and increasing distributed capacitance per meter.
How is coaxial capacitance per meter calculated?
Capacitance per unit length is calculated using C = 2pi epsilon_0 epsilon_r / ln(D/d) approximately 55.63 * epsilon_r / ln(D/d) pF/m. Lower D/d ratios increase capacitance.
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.