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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.

 

Click here for popular coax cable properties.

 

Flexible braided coax coaxial cable drawing - RF Cafe Most often used braided flexible (shown at left) and solid tube semi-rigid (shown at right) types are listed here. Semi-rigid coax coaxial cable drawing - RF Cafe

 

Capacitance (C) = Coaxial cable capacitance equation - RF Cafe

Inductance (L) = Coaxial cable inductance equation - RF Cafe

Impedance (Z0) = Coaxial cable impedance equation - RF Cafe

Velocity = Speed of light in a vacuum equation - RF Cafe (c = speed of light in a vacuum)

Cutoff Frequency =

Reflection Coefficient = Coaxial cable reflection coefficient equation

VSWR = Coaxial cable VSWR equation

Peak Voltage = Coaxial cable peak voltage equation

, 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.

(NOTE: f=frequency in GHz)

Coaxial cable attenuation equation conductor & dielectric - RF Cafe

ρr = 1 for copper, 10 for steel

Coaxial cable attenuation chart graph - RF Cafe

ρrd = inner conductor material resistivity relative to copper
ρrD = outer conductor material resistivity relative to copper
δ = loss tangent

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