Cable Impedance Calculator
Calculate the characteristic impedance of coaxial and parallel cables.
Enter conductor diameter, spacing, and dielectric constant to find impedance in ohms.
Characteristic impedance is the inherent impedance of a transmission line — the ratio of voltage to current for a wave traveling along the cable. When cable impedance is mismatched with connected devices, signal reflections occur, causing signal loss, interference, and in RF systems, reduced power transfer.
Why Impedance Matching Matters
In RF and high-frequency systems, impedance mismatch causes reflected power. The VSWR (Voltage Standing Wave Ratio) measures this mismatch:
- VSWR of 1:1 = perfect match, all power transferred
- VSWR of 2:1 = ~11% power loss from reflection
- VSWR of 3:1 = ~25% power loss
In audio systems, impedance mismatch affects frequency response and signal level but rarely causes reflections due to the low frequencies involved.
Coaxial Cable Impedance Formula
Z = (138 / √εr) × log₁₀(D / d)
Where:
- Z = characteristic impedance (ohms)
- εr = relative permittivity (dielectric constant) of insulation
- D = inner diameter of the outer conductor (shield)
- d = outer diameter of the inner conductor (center wire)
Common Dielectric Constants (εr)
| Dielectric | εr |
|---|---|
| Air / vacuum | 1.0 |
| PTFE (Teflon) | 2.1 |
| Polyethylene (PE) | 2.3 |
| PVC | 3.4 |
| Glass | 6–10 |
Standard Cable Impedances
- 50 Ω: RF, antenna, instrumentation (RG-58, RG-213, SMA connectors)
- 75 Ω: Video, cable TV, satellite (RG-6, RG-59, F connectors)
- 93 Ω: Some computer applications (RG-62, ArcNet)
- 600 Ω: Balanced audio lines (XLR, telephone)
Parallel Wire Formula
For two parallel conductors of diameter d, separated by center-to-center distance D:
Z = (276 / √εr) × log₁₀(2D / d)
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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