Custom Search


More than 12,000 searchable pages indexed.  
Your RF Cafe


5th MOB: Airplanes and Rockets: Equine Kingdom: 
•−• ••−• −•−• •− ••−• •
RF Cafe Morse Code >Hear
It<


Job Board 

About RF Cafe™ 

Sitemap 
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.
Braided Flexible Coax
SemiRigid 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
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
Capacitance (C) 
Note: a and b can be in any units of length as long as they are both the same. 
Inductance (L) 
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. 
Impedance (Z_{0}) 
Note: a and b can be in any units of length as long as they are both the same. L has units of Farads and C has units of Henries. 
Speed of Light (%c) 
Note: V has the same length units as c. 
Cutoff Frequency 
Note: a and b must be in units of length shown. 
Reflection Coefficient & VSWR 
Note: Γ is unitless. VSWR is written as a VSWR:1 ratio. 
Attenuation 
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