This coaxial cable impedance calculator finds the characteristic impedance Z₀ from the inner and outer conductor diameters and the dielectric constant. Enter the values above.
Formula
Z₀ = (138 ÷ √εr) × log₁₀(D ÷ d)
- D — outer (shield) inner diameter, d — inner conductor diameter, εr — dielectric constant
Example
D = 7.25 mm, d = 2 mm, εr = 2.3: Z₀ = (138 ÷ √2.3) × log₁₀(3.625) = 50.9 Ω.
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Frequently Asked Questions
How do I calculate coaxial cable impedance?
Use Z0 = (138 / sqrt(er)) x log10(D/d), with the conductor diameters and the dielectric constant.
What is a standard coax impedance?
50 ohms for RF and test gear, and 75 ohms for video and antenna feeds.
Does the dielectric affect impedance?
Yes. A higher dielectric constant lowers the impedance for the same conductor geometry.