Sources of losses in Transmission lines

The three major sources of losses in RF transmission lines are:

1. Copper Losses:-

(a)    I2 R Losses. This is due to the resistance associated with the conductors constituting the transmission lines or in other words this loss is due to power dissipation as a result of heating in pure resistance .This loss is frequently dependent and increases with increase in frequency .[Resistance associated with transmission line cannot be measured with an ohmmeter because at radio frequencies,RF ,the current tends to flow on the conductor surface  or very near to the conductor surface due to effect called skin effect.

(b)   Crystallisation. Copper losses increases due to ageing of transmission lines. Losses are more when the line is subjected to high temperature, high winds and moisture. Moreover bending of the line back and forth causes the line to become brittle and hence cracks appear. This effect is known as crystallisation of conductors.

2. Dielectric losses:- if the dielectric between the conductors has a finite conductivity, an energy loss will occur and appears in the form of heat in the dielectric material and increases with increase in frequency .These losses are also proportional to the voltage across the dielectric .With increased frequencies, solid frequencies dielectric properties worsen and hence the loss.

3. Radiation losses:- The passage of high –frequency current through an open –wire line gives rise to an electromagnetic field surrounding the conductors and causes RF power to be radiated into free space or nearby circuits. These losses are high when spacing between the lines is high as the transmission line act as an antenna. Thus, Radiation losses are more in parallel wire lines than in coaxial line.

Although transmission lines are not lossless, but for all practical purposes, they can be assumed to be lossless. This assumption is valid in case of practical transmission lines. The conductors are normally made up of an easily machinable metal with a coating of silver or any other highly conducting material. It does not matter what that inner material is at radio frequency, RF and microwave frequencies ,the current tends to flow only near the surface . This reduces copper loss. Also in air filled transmission lines and lines using loss dielectric like quartz, Teflon, dielectric loss will also be negligible and in shielded lines, radiation loss is negligible.

Also ‘α’ for most low-loss ultra high frequency, UHF gives satisfactory results. Because as transmission lines are ordinarily a few wavelengths long l, at most and α l can usually be neglected compared with β l at ultra high frequency, UHF. therefore for many purpose low-loss lines may be treated as though they are lossless . Therefore if


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