A coaxial cable is a cylindrical transmission line consisting of two conductors sharing a common axis along the cable length and an insulator material separating them. The conductor running in the center of a coax cable is called the core, which usually is made from a solid copper wire. Then the core is covered with a plastic layer of insulation dielectric that maintains electrical separation with the other conductor. The outer conductor, called the shield is usually a braid of thin metallic strands so that the whole conductor is flexible. However, there are other constructions of coaxial cables where the shield is semi rigid or completely rigid depending on the application requirement. The final cover acts like a protective jacket from physical damage as well as unwanted electrical contacts.
Figure 1 – Coaxial cable physical detail
Coaxial cables are ideal for economical transmission of electrical signals in the radio frequency range with less external electromagnetic interference. Depending on the material used to manufacture the cable, it will have a characteristic resistance to the flow of current called impedance. The most commonly available coaxial cables are with 50 or 75 ohm characteristic impedance. And depending on the shielding material quality the cable will have a shielding effectiveness characteristic measured in decibels. Higher negative shielding effectiveness dB values indicate the cable’s less susceptibleness to signal leaking outside the cable or exterior interference penetration. The common areas where coaxial cables are widely used is for interconnection of home entertainment devices, data network endpoints, telecommunication equipment, antennas to their transceivers and security cameras to monitors.
The core of the cable carries the signal while the shield is connected to a ground circuit. This will direct intruding and leaking energy to be neutralized around the shield. Any kind of damage on the shield might result in creating an opening of the shield surface which in return causes significant signal quality defect. Therefore, due to the physical nature of coaxial cables, there are certain technical cares that need to be considered during their installation and use. Some of the basic points are;
- Joining points need connectors designed for the specific use on coaxial cables.
- The bending radius is limited to 10 times the outside diameter of the whole cable.
- Splices should be clean from burrs and nicks to avoid shielding deterioration.
- Dummy loads that match the cable length, impedance and application frequency need to be used.