Coaxial Cables have four layers

• Centre Conductor
• Dielectric
• Screen
• Overall Jacket

Main Characteristics of Coaxial Cable

Characteristic Impedance- The entire CATV system works on 75 ohms impedance.Therefore all the equipments forming part of CATV including Coaxial Cables should be of 75 ohms + 3 ohms. By impedance we mean the opposition to flow of signals due to inherent capacitance and inductance of cable. In case any of the equipment including Coaxial cable has a difference of impedance it will create impedance mismatch and would create problems in the entire CATV system i.e. performance is affected.

Attenuation -By attenuation we mean Transmission Loss. This transmission loss occurs due to

• Type of material used in the construction of Coaxial Cable
• Cross sectional area of centre conductor and
• Length of the cable.

Therefore, raw material used in the construction of coaxial cable should be of superior quality. Secondly more the cross-sectional area of the centre conductor lesser will be the attenuation. Thirdly, on the contrary attenuation also increases in proportion to an increase in the length of the cable.

Structural Return Loss

This parameter should be given utmost importance during procurement of Coaxial cable because if structural return loss of a Coaxial Cable is not good, then investment in the same might be wasted. Structural return loss means a ratio of reflected power in dB (decibals) to the incident power from a particular point in a cable i.e. it is a measure of how much signal is reflected as a result of impedance mismatch. For example, if the impedance of Coaxial cable is correct i.e. 75 ohms and if the length of the same is cut into pieces, then the impedance of the cut pieces should also be 75 ohms and it should not vary. This means that impedance should not vary with the length good quality cable.