About Coaxial Cable

By Shoaib Khan

A coaxial cable is used to transmit signals, such as video, radio and data. The cables are round and of a uniform radius from end to end. Although most people are only familiar with coaxial cables that are used to establish cable TV connections, there are many different variants that are extensively used in measurement devices, audio equipment, ethernet networks and telecommunications.

Internal Structure

Inside a coaxial cable, a single copper wire is in the center, surrounded symmetrically by a braided (meshed or interlaced) conductor shell on all sides. An insulating diaelectric layer separates the copper wire in the center from the braided copper mesh. The diaelectric is an important element in the structure and plays an important part in giving fundamental characteristics to the particular type of coaxial cable. Physical properties like permeability and permittivity influence the cable's characteristics, such as inductance and capacitance. These values influence the speed with which electrical data can be transmitted through the wire.

Transmission

In a coaxial cable, data is carried through the copper wire in the center of the cable. The outer meshed layers represent the line to the ground. Both of the conducting layers in the cable share the same axis, hence the term "coaxial." Transmission of data through coaxial cables is affected by properties of the cable and of connected devices.

Impedance

Electrical impedance refers to the level of opposition to an alternating current (AC), and coaxial cables, like all electrical components, have a characteristic impedance. This measure is affected by physical and structural attributes of a particular coaxial cable type, especially characteristics of the diaelectric material, the thickness (radii) of the conducting materials, and the size and spacing of conductor layers. The length of a cable has no effect on its impedance.

Noise

A coaxial cable and systems connected through it are not perfect. This is because, no matter the type, coaxial cables are always susceptible to signals, or "noise" radiating from them. The outer conducting layer acts as a barrier that restricts interference by limiting the signal coupling with signals from adjacent cables. The higher the level of restriction, the less the outward radiation of energy.

Applications

Common coaxial cable impedance is between 50 ohms and 75 ohms. Generally, 50-ohm coaxials are used in applications like antenna connections for radio transmitters, devices for measurement, and in data communications, such as networking through ethernet cabling. Seventy-five-ohm cables are mostly used to transmit video signals, antenna signals for TV and some digital audio signals.

Customization

Since a coaxial cable's properties are entirely dependent on its constituent parts, cables with different impedances can be designed for specific scenarios. For most applications, using standardized cables is much more practical because they are cheap, widely available and very efficient. However, for applications that demand properties outside the scope that normal cables can provide, using a tailor-made cable will give optimum results. Impedance between 30 ohms and 44 ohms tends to handle maximum power, whereas 77 ohms is ideal for limiting loss, and 90 ohms gives the lowest capacitance (per foot). Coaxial cables with higher impedances than 90 ohms are very rare.