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Transmission Lines

A transmission line, in general refers to a physical system use to transmit certain physical quantities. In a electrical engineering terminology, transmission lines are a means or two transmit electrical signal or power between two distinct points which are at identifying distance apart.

Transmission line while used to transmit energy, also compromise certain circuit elements like filters, indicators, capacitors, resonant circuits and even insulators. Transmission lines in essence consist of a collection of a wire or a set of conductors made up of good conductors of electricity, such as copper, aluminium etc.,

Performance of transmission line depends upon those factors such as insulation between two adjacent conductors frequency power transmission amount of power that is transmitted etc.

There exist four basic types of transmission lines viz. power lines, open-wire line, paired cable, co-axial cable.

Matching of Transmission Lines

Matching on transmission line: A transmission line is connected between a transmitter and an antenna or between antenna and a receiver, which acts like a communication link. It is the transmission line that decide the efficiency of power transfer.

In communication networks, it is necessary that elements of the network should be designed such that maximum power transfer takes place between the source to load. This is governed by maximum transfer theorem.

For maximum power transfer between source and load, the load resistance should equal to that of source but of opposite sign. At the higher frequencies, the imperdance transformers can be used which consists of T-sections in the various arrangement. Some of the arrangements include,

1) Quarter wave transformer
2) Stub matching

Types of Communication Lines

Communication lines are used for transmission of small quantity of power at band of frequencies. Three types of communication lines extending from audio frequencies to high frequencies are:

1. Open wire line
2. Paired cable
3. Co-axial cable

Open wire line:

It consist of two wires of sufficient cross sectional area, which are running on insulators firmly and rigidly supported by cross arms put at certain height from ground by means of certain structures. The conductors which runs parallel, have air as dielectric.

Paired cable:

It consists of large number of pairs of wires each pair consisting of two wires which are insulated from each other by oil impregnated insulator paper.

Co-axial cable:

It consists of one solid conductors of less diameter actually inside another hollow conductor of large diameter and of certain thickness.

Eliminating Phase and Frequencies and Distortions in high quality transmission lines

Equalizer can be used in cascade with transmission lines in order to remove frequency distortion. Now total attenuation becomes constant over desired frequencies band. The increase attenuation can be overcome by cascading large gain amplifiers to transmission lines. In essence, characteristics are adjusted to be inverse to those of the lines, resulting in an overall frequency response over the desired frequency bands.

For picture signal transmission, delay equalizer are use to eliminate delay distortion.

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