Coaxial cable is a shielded one-conductor cable with the conductor embedded in a di-electricum, which is wrapped in foil or a braided shield. It is called coaxial because it includes one physical channel that carries the signal, surrounded by a layer of insulation and by another concentric physical channel, both running along the same axis.
The attenuation (power loss) of coax cables is defined by the loss in dB per unit length at a given frequency. The loss rises with the length of the cable and with the frequency. But with rising frequency the actual loss is not linearly dependent upon the frequency.
The loss is added by 1) resistive loss, 2) dielectric loss and 3) radiated loss.
The resistive loss within the cable depends on conductor resistance and flowing current. The loss results in heat being dissipated. The resistive loss increases as the square root of the frequency. The area through which the current flows in the conductor is limited by the skin effect. With AC currents flowing through cables at higher frequencies, the skin effect is coming more and more into account the higher the frequency is. Cables carrying high frequency signal act as low pass filters. The resulting loss in the high frequency part is called insertion loss. To prevent this often multi-stranded conductors are used respectively the conductive area of the cable are increased resulting in larger diameters.
The dielectric loss increase linearly with frequency but is independent of the size of the cable. The dielectric loss also dissipates as heat.
The radiated loss is the least important because only a very small amount of power is radiated from the cable.
As resistive loss increases as the square root of frequency and dielectric losses increase linearly, the resistive loss dominates at lower frequencies and the dielectric loss dominates at higher frequencies.
Because of the insertion loss of the cable digital video signals must be recovered after being transmitted over longer cable runs. This can be performed by pre-emphasis at the start point of the cable or by adding equalization to the receiver at the end of the cable. For the use of pre-emphasis the cable characteristics and the cable length must be known. Equalization can be done with adaptive cable equalizers that is controlling the amount of equalization by a feedback loop through the cable. With active equalization systems not only the signal but also noise is amplified. Pre-Emphasis and Equalization is done by increasing the high frequency part according the insertion loss of the cable to finally get a more or less linear signal again.
|1.0-2.3 Mini Coax Connector 75 Ohm||Video, digital Video|
|1.6-5.6 Coax Connector 75 Ohm||Video, digital Video|
|BNC||Video, digital Video|
|TNC||Video, digital Video|
|Triax||Video, digital Video|
|Composite PAL||Component SMPTE 259M||Component
|Bandwidth:||143 Mb/s||177 Mb/s||270 Mb/s||360 Mb/s||1.5 Gb/s|
|= Diameter of Inner Copper Wire / Diameter of Insulation|