Coaxial Cable Impedance

Formula

Description

The characteristic impedance of a coaxial cable depends on the ratio of the outer conductor inner diameter to the inner conductor outer diameter, and the dielectric constant of the insulating material between them. Standard impedances are 50Ω (used in most RF systems and test equipment), 75Ω (used in video, CATV, and satellite systems), and 93Ω (used in some older data transmission). The 50Ω standard represents a compromise between minimum attenuation (77Ω in air) and maximum power handling (30Ω in air) for the same outer conductor size.

Variables

• Z0 — Characteristic impedance (Ω)
• εr — Relative dielectric constant of the insulating material
• D — Inner diameter of the outer conductor (shield) (mm)
• d — Outer diameter of the inner conductor (center conductor) (mm)

Practical Notes

Common dielectrics: solid PE εr = 2.25, foam PE εr = 1.5, PTFE εr = 2.1, air εr = 1.0. RG-58 (50Ω): d = 0.9mm, D = 2.95mm, PE dielectric. RG-6 (75Ω): d = 1.0mm, D = 4.7mm, foam PE. Lower εr allows faster signal propagation (higher velocity factor) and lower loss.