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. Be very careful to realize that at frequencies far from DC, factors like skin depth and effective inner and outer conductor diameters may be significantly different than the physical measured values, and that can significantly affect real world results. Therefore, be sure to consult manufacturers’ published data before making a final decision. I leave it to other sources to provide the complex equations needed to precisely model coaxial cables.
Braided Flexible Coax
a = outside radius of inner conductor (inches)
b = inside radius of outer conductor (inches)
c = speed of light in a vacuum = 299,792 km/s = 186,282 mi/s
0 = permittivity of free space = 8.85419×10 -12 F/m
r = relative dielectric constant
Note: a and b can be in any units of length as long as they are both the same.
Note: a and b can be in any units of length as long as they are both the same. However, l must be in the units shown.
Note: a and b can be in any units of length as long as they are both the same. C has units of Farads and L has units of Henries.