Coax Impedance Calculator
Calculate the characteristic impedance of a coaxial cable from its dimensions and dielectric constant.
mm
mm
Characteristic Impedance
53.7 Ω
Velocity Factor0.659
Diameter Ratio3.89
Characteristic Impedance vs Outer Conductor ID
Coaxial Cable Impedance
Formula
Z0 = (138 / sqrt(Er)) x log10(D/d)
Where D is the outer conductor inner diameter, d is the inner conductor outer diameter, and Er is the relative dielectric constant.
Common cables: RG-58 (50 ohm), RG-59 (75 ohm), RG-6 (75 ohm).
Example Calculation
RG-58-like: D = 3.5 mm, d = 0.9 mm, PTFE dielectric (Er = 2.3).
- 01Ratio = 3.5 / 0.9 = 3.889
- 02Z0 = 138 / sqrt(2.3) x log10(3.889)
- 03Z0 = 91.0 x 0.5898 = 53.7 ohms
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