Coaxial Cable
Understanding Coaxial Cable
Coaxial cable is the most widely used RF transmission line. Its closed geometry provides excellent electromagnetic shielding, preventing both radiation from the cable and pickup of external interference. The characteristic impedance is determined by the ratio of outer to inner conductor diameters and the dielectric constant of the insulating material.
Cable Types
- Flexible cable (RG series): Stranded center conductor with braided shield. RG-58 (50 ohm), RG-59/RG-6 (75 ohm). General purpose, moderate loss.
- Semi-rigid cable: Solid outer conductor (copper or stainless steel tube). Superior shielding and phase stability. Used in precision test setups and military systems.
- Conformable cable: Corrugated outer conductor that can be hand-formed. Compromise between semi-rigid performance and flexible convenience.
- Low-loss cable: Specialized designs (LMR-400, Ecoflex) with foam dielectric for reduced attenuation in long runs.
Z0 = (138/sqrt(er)) x log10(D/d)
Attenuation: increases as sqrt(f) for conductor loss
Cutoff frequency: fc = c/(pi x (D+d)/2 x sqrt(er))
Example: RG-58 (50 ohm)
Loss at 1 GHz: 17 dB/100ft
Loss at 5 GHz: 38 dB/100ft
Frequently Asked Questions
What is coaxial cable used for in RF?
Coaxial cable connects RF components (antennas, amplifiers, test equipment) while containing the electromagnetic field within its structure. It provides shielded, impedance-controlled signal transmission from DC to tens of GHz.
Why does coaxial cable loss increase with frequency?
Loss increases because of the skin effect: at higher frequencies, current flows in a thinner layer on the conductor surface, increasing effective resistance. Dielectric loss also increases linearly with frequency.
What is the maximum frequency for coaxial cable?
Depends on cable size. Standard SMA cables work to 18-27 GHz. 2.92mm (K connector) cables reach 40 GHz. 1.85mm cables reach 65 GHz. Above these frequencies, the cable supports unwanted higher-order modes.