# Coaxial Cable Impedance Calculator

Most RF Coaxial Cables have an impedance of 50 ohms or 75 ohms. These are considered to be standardized impedance values with cables readily available. In some cases, users require a custom impedance value. This can be achieved by regulating the inner and outer diameter of the coaxial cable, along with the dielectric.

Note: The unit of the diameter of the outer and inner conductor must be the same for this calculator to work. They are always in a ratio so will cancel each other out.

### Result

• Capacitance per unit length (C)
pF
• Inductance per unit length (L)
nH
• Impedance per unit length (Z)
Ω
• Cut-off Frequency (fc)
GHz  Capacitance of Coax Cable

The coax cable has a capacitance as a result of the gap between inner conductor and outer shield of the cable. The capacitance value varies with the spacing of the conductors, the dielectric constant, and the impedance of the cable. This capactiance can be calculated by the following formula. Where:

C = Capacitance in pF/Meter
εr = Relative Permeability of the dielectric between the inner and outer conductors
D = Outer Diameter
d = Inner Diameter

Inductance of Coax Cable

The inductance of the coaxial cable is proportional to the length of the line and does not depend on the dielectric constant of the material between the conductors. The inductor can be calculated by the following formula. Where:

L = Inductance in µH / metre

D = Outer Diameter

d = Inner Diameter

Note: The value of μr in this calculator has been taken as 1.

Coax Impedance Calculation

The impedance of the RF coaxial cable depends on the dielectric constant of the cable and the diameter of the inner and outer conductors. The impedance can be calculated by the following formula. Where:

Zo = Characteristic impedance in Ω

εr = Relative permeability of the dielectric

D = Inner diameter of the outer conductor

d = Diameter of the inner conductor

Note: The units of the inner and outer diameters of the conductor can be in any unit as they are always in a ratio so will cancel each other out.

Cutoff Frequency of Coax Cable

The cutoff impedance of the coaxial cable depends on the Relative Permittivity of the medium and both the outer and inner diamemter of the conductors. The cutoff frequency can be calculated by the following formula. Where:

εr = Relative permeability of the dielectric

D = Inner diameter of the outer conductor

d = Diameter of the inner conductor