Coaxial Cable Characteristic Impedance Calculator
Enter conductor dimensions in the same unit (mm, in, etc.) and the dielectric constant of the insulator.
where d = inner conductor diameter, D = inside diameter of shield, and εr = relative permittivity.
What this coax impedance calculator does
This tool computes the characteristic impedance of a coaxial cable from geometry and dielectric material. Characteristic impedance (Z0) is one of the most important cable parameters in RF, video, instrumentation, and high-speed digital systems. If your cable impedance does not match your source and load, reflections appear, and signal quality can degrade quickly.
In addition to Z0, the calculator also estimates:
- Velocity factor (how fast signals travel relative to the speed of light)
- Capacitance per meter
- Inductance per meter
- Optional total capacitance, total inductance, and one-way delay for a specified length
The key formula behind coax impedance
Characteristic impedance equation
For a standard coax with non-magnetic dielectric (μr ≈ 1), impedance is:
Z0 = (60 / √εr) × ln(D/d)
- d = diameter of center conductor
- D = inner diameter of the outer conductor/shield
- εr = relative permittivity of dielectric
- ln = natural logarithm
Because the equation uses the ratio D/d, you can use any consistent length unit. Just ensure both diameters are entered in the same unit.
How to use the calculator correctly
Step-by-step
- Measure or specify the center conductor diameter (d).
- Measure or specify the shield inner diameter (D).
- Enter dielectric constant εr of the insulator.
- (Optional) Enter cable length to get delay and total L/C estimates.
- Click Calculate.
Common εr values: air ≈ 1.0006, foam PE ≈ 1.3–1.6, solid PE ≈ 2.25, PTFE ≈ 2.1.
Typical design targets: 50 Ω vs 75 Ω
50 Ω coax
The 50 Ω standard is common in RF power and instrumentation systems because it offers a practical compromise between power handling and attenuation.
75 Ω coax
75 Ω is widely used in video, CATV, and many receive-only signal chains because of low attenuation characteristics.
Choosing between 50 Ω and 75 Ω is usually system-driven: match your equipment, connectors, and transmission line standard first.
Worked mini-example
Suppose you have d = 1.0 mm, D = 3.5 mm, and εr = 2.25. Then:
- D/d = 3.5
- ln(3.5) ≈ 1.2528
- 60/√2.25 = 40
- Z0 ≈ 40 × 1.2528 = 50.11 Ω
That geometry is very close to a 50 Ω cable design.
Common mistakes that cause wrong results
- Using different units for d and D (for example mm for one and inches for the other)
- Confusing outer cable diameter with shield inner diameter
- Using an incorrect dielectric constant for foamed material
- Ignoring manufacturing tolerances and braid/shield realities in practical builds
This calculator gives an idealized result; real cable performance also depends on conductor finish, braid coverage, dielectric uniformity, frequency, and losses.
Quick FAQ
Does frequency affect characteristic impedance?
In ideal TEM coax, Z0 is mainly geometric/material. Practical measured impedance can vary slightly with frequency due to dispersion and loss effects.
Can I use radii instead of diameters?
Yes. Since the equation uses a ratio, using D/d with diameters is equivalent to using b/a with radii, as long as both are consistent.
Is this suitable for precision RF production?
It is excellent for design estimation and education. For production, use full EM modeling and confirm with TDR/VNA measurements.