Cable Impedance Calculator

What Is Cable Characteristic Impedance?

Characteristic impedance is the effective AC resistance a transmission line presents to a signal. Matching source, cable, and load impedance reduces reflections and signal loss. Coaxial and twisted-pair cables use geometry and dielectric material to set this value.

Impedance Formulas

Coaxial cable:

$$\text{Impedance} = \frac{60 \times \ln(D/d)}{\sqrt{\varepsilon_r}}$$

Twisted pair:

$$\text{Impedance} = \frac{120 \times \ln(2s/d)}{\sqrt{\varepsilon_r}}$$

Here $d$ is inner conductor diameter, $D$ is inner shield diameter, $s$ is wire spacing, and $\varepsilon_r$ is the dielectric relative permittivity. Common coax values are 50 Ω and 75 Ω.

Frequently Asked Questions

Why is 50 Ω common for RF coax?

50 Ω balances low attenuation and power handling for many RF applications. Video coax often uses 75 Ω for lower loss at high frequency.

What dielectric value should I use?

PTFE is about 2.1, polyethylene about 2.3, and foam dielectrics can be lower. Check the cable datasheet for exact $\varepsilon_r$.

How is propagation delay calculated?

Signal speed is $c / \sqrt{\varepsilon_r}$. Delay equals cable length divided by that velocity.

What is coax cutoff frequency?

It is the frequency where higher-order wave modes can propagate. Below cutoff, only the TEM mode is supported in ideal coax.

Do I need twisted pair or coax inputs?

Coax needs inner and shield diameters. Twisted pair needs conductor diameter and center-to-center wire spacing.

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What Is Cable Characteristic Impedance?

Impedance Formulas

Frequently Asked Questions

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