Coaxial cable

Overview

Coaxial cable or coax, is an electrical cable consisting of an inner wire covered by an insulating sheath and a conductive shield. The cable is used as a data transmission line in many fields, including internet connections, radio receiver connections, and cable television. The benefit of coaxial cables is low susceptibility to electromagnetic interference due to the shielding.

Technical Details

Coaxial cable consist of 4 layers. A central wire core is the primary conductor. It is surrounded by a dielectric insulator crafted of a semi-rigid plastic. This is surrounded by a braided conductive shield, then coated with a plastic jacket.

Coax is constructed to operate optimally under radio frequencies. The inner wire functions as an antenna, causing a power loss. The tube-shaped shield surrounding the core acts as an RF shield, confining the waves to be confined to the space inside the tube, and back to the core. It maintains the proper electric potential by being connected to Ground. To function properly, the distance between the center core and shield must be maintained. The insulator serves to keep the conducting components properly spaced, and a distortion of this causes radio reflections in a transmission hindering effect (standing waves).

Transmission

As a transmission line, coax has well defined characteristics describing its behavior. The transmission lines have resistance over the length of the wire, and the dielectric acts as a capacitor, with the combined effects of impedance, defined by:

Capacitance
<math>C = {2 \pi \epsilon \over \ln(D/d)}= {2 \pi \epsilon_0 \epsilon_r \over \ln(D/d)}</math>

• in farads per meter

Inductance
<math>L = {\mu \over 2 \pi} \ln(D/d)= {\mu_0 \mu_r \over 2 \pi} \ln(D/d)</math>

• in henrys per meter

Where "d" is the diameter of the inner conductor, "D" is the inside diameter of the shield, <math>\epsilon</math> is the dielectric constant of the insulator, and <math>/mu</math> is the magnetic permeability of the insulator.

Connecting

Coax cable is defined in terns of impedance based on the materials used. The most common impedance values are 50, 52, 75, and 93 Ohms.

Cables are connected with RF connectors, which are specifically designed to have the same impedance as the cable itself.