Braided Flexible Coax
SemiRigid Coax
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 welldefined 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.
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
ε = dielectric constant = ε_{0}
* ε_{r}
ε_{0} = permittivity of free space = 8.85419x10^{12}
F/m
ε_{r} = relative dielectric constant
μ_{r} = relative permeability
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.

Note: V
has the same length units as
c.

Note: a
and b
must be in units of length shown.

Note: Γ
is unitless.
VSWR is written as a
VSWR:1
ratio.

Equations for coaxial cable attenuation used to be offered here, but while redesigning
this page and attempting to verify the equations, I discovered (or probably rediscovered)
that theoretical values versus published measured values for realworld cable varied
a lot at every frequency. RG6 coax, for example, can have a loss at 1 GHz ranging
from a little over 5 dB/100 feet to nearly 10 dB/100 feet, depending
on the dielectric type, actual conductor and dielectric diameters, and it seems very
importantly, the construction of the outer shield conductor. A single layer of loosely
woven braid versus a one or more dense layer(s) of braid and one or more layer(s) of
metal foil versus semirigid versus hardline coaxial cable makes calculation using a
simple, onesizefitsall equation impossible.
Therefore, I have removed the equations I used to have and instead recommend that
you visit coaxial cable manufacturers'
websites and consult their published data and decide what value of attenuation per
foot, meter, etc., is most appropriate for your realworld applications.

Related Pages on RF Cafe  Coaxial Cable
Specifications  Capacitor Dielectrics & Descriptions
 Dielectric Constant, Strength, &
Loss Tangent  Conductor Bulk Resistivity & Skin
Depths  Coaxial Cable Equations 
Coaxial Cable Specifications 
Coaxial Cable Vendors  Coaxial Resonator  Skin Depth Calculator

Coaxial Connector Usage Chart
