“Characteristic impedance” is generally implied when speaking of the impedance of a cable, connector, or cable assembly. Maximum power transfer and minimum signal reflection occurs when the characteristic impedance of a cable assembly matches that of the other components in the system. If all the impedances match, losses are due only to the attenuation of the transmission line: otherwise there will be additional reflection losses. The characteristic impedance (Zo) is directly related to the ratio of the inner and the outer conductor diameters, and inversely related to the dielectric constant of the core material (ε) . Due to the “skin effect” of RF energy transfer, the important dimensions are the outer diameter of the center conductor, d, and the inner diameter of the outer conductor, D.
Zo (Ohms) = (138/√ε ) x (log D/d)
A 50-Ohm (Ω) standard has been chosen as the characteristic impedance of most commonly available components. While other impedances might offer better characteristics in particular situations (for instance, 75 Ohm for lowest attenuation, 35 Ohm for best power handling), the need for non-standard characteristic impedances small in most applications. Components with impedances other than 50 Ohm, from 35 Ohm to 185 Ohm, are available but normally at a higher cost.