Coaxial Cable Equations

Most professional engineers and technicians will never have the need to calculate the capacitance, inductance, or impedance of a coaxial cable since they are usually designing systems using well-defined components that are manufactured to exacting specifications. Students, hobbyists (Ham radio operators), and research types are probably the ones most likely to actually plug numbers into a calculator. For those people, I present these equations.

 

Most often used braided flexible (shown at left) and solid tube semi-rigid (shown at right) types are listed here. Click here for popular coax cable properties.

Capacitance (C) = Inductance (L) = Impedance (Z0) =   Velocity =   %c (speed of light in a vacuum) Cutoff Frequency = (here is a calculator) Reflection Coefficient = VSWR = Peak Voltage = , where d = outside diameter of inner conductor in inches D = inside diameter of outer conductor in inches S = maximum voltage gradient of cable insulation in volts/mil ε (εr) = relative dielectric constant K = safety factor f = frequency in MHz Cable attenuation is the sum of the conductor losses and the dielectric losses per the following equations. ρr = 1 for copper, 10 for steel , where d = outside diameter of inner conductor in mm D = inside diameter of outer conductor in mm εr = relative dielectric constant f = frequency in GHz ρrd = inner conductor material resistivity relative to copper ρrD = outer conductor material resistivity relative to copper δ = loss tangent

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