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Transmission line is defined as the path of carrying alternating electrical energy from source to load.
For example the wire used between T.V antenna and television set or the wire used between transmitter antenna and transmitter are known as transmission lines.
Types of Transmission Lines
There are the following types of transmission lines.
1. Balance Two Wire Line
As shown in the given diagram the two wire transmission lines consists of the figures A,B and C. each construction for the given figure is given as below.
- In this type of construction for two wire transmission lines the insulated spacers are used in order to maintain the distance between the transmission lines or between the two conducting wire equally throughout.
- In this type of transmission line the two conducting wires are kept parallel to each other with the help of plastic material. This is used throughout between the conducting wires.
- In this type of transmission line the rubber piping is used in circular rectangular or square shape. The two conducting wires are kept inside the rubber at opposite sides of the piping. These conducting wires urn throughout the construction and remains parallel to each other.
There are the following merits of balance two wire lines.
- The cost of two wire transmission line is very low as compared to other types of lines.
- To design the open two line transmission line is quite simple and easy too.
- Open two wire lines are capable of handling high power.
- The external interference of the signal in open two wire lines is more as compared to other types of transmission lines.
- Due to external interference the output at the load end of two wire transmission line will be noisy.
- To use the two wire transmission lines in the twisty paths is quite difficult.
- Formula for finding out the characteristics impedance of open two wire transmission line is given below.
- It cannot be used on very high frequencies because it will generate skin effect.
Zo = 276 log10 2D/d
Here d stands for the diameter of the wire.
D stands for the distance between two wires from its aerials.
Zo stands for characteristics impedance
2. Co-axial Cable
As shown in the given diagram the co-axial cable consists of inner conducting wire made of copper, over this conducting wire the coating of polyethylene or taplon material is carried out. Then it is enclosed in the braded wire in the shape of mash. The outer surface of this wire is enclosed in a plastic jacket.
There are the following merits of co-axial cable.
- As the outer conductor (braded wire) is grounded, therefore the possibility of external interference is minimized. The output of the load end will be less noised.
- The co-axial cable is used for high frequencies transmission.
- This type of transmission cables can be easily used if the path of energy from source to load is twisty or complicated.
- Co-axial cable occupies less space as compared to two wire lines.
- The conductor which carries the energy from source to load is protected from dust, rust etc. due to proper insulation.
There are the following demerits of the co-axial cable used as transmission line.
- This type of transmission line is costly with respect to two wire lines.
- Designing of co-axial cable is difficult as compared to two wire lines.
- This type of transmission lines handles low power transmissions.
- Formula for the impedance is
Zo = 233 log10 D/d
Where Zo stands for characteristics impedance.
D stands for internal wire diameter of the braded wire.
d stands for diameter of the inner conductor.
3. Micro strip Line
A microstrip is simply a copper track running on a side of the PCB while the other side is plain groun plane. The formula will give you the charactersistic impedance of the track, as well as the effective dielectric constant based on the geo-metric parameters. The table provides usual values for 1.6 and 0.8 mm thick PCBs as well as for the standard FR4 substrate or the most advanced Rogers R04003.
As shown in the above diagram the micro strip consists of a conducting plate made of copper which works as an earth plate in the circuit. There is thick coat of insulating material over the copper plate which is made of fiber glass or polystyrene. This insulated plate works as a dielectric in the micro strip line. At the top of the insulated plate one or more than one strips of the best conducting material are plated which is made of gold, aluminum etc.
There are the following merits of the micro strip line.
- Very high frequency.
- Small size
- Low weight.
- Losses are minimum.
- This type of transmission line is used for very high frequency.
- Micro strip lines are used in integrated circuits where distance between load and source is very short.
- As the path of energy is made of very good conductor like gold, therefore the losses of energy are minimum possible.
- The weight of micro strip line is low.
There are the following demerits of micro strip line.
- The cost of micro strip is very high as compared to co-axial and two wire line.
- The micro strip line cannot be used as a transmission line when the distance between source and load is long.
- This type of transmission line cannot be used in twisty paths between source and load.
Losses of Transmission Line
There are the following losses of transmission line.
When the high frequency current or voltage wave form flow through the transmission lines, the magnetic field expands and collapse around the transmission lines at the same rate of input frequency. As we know that around the magnetic field there is also an electric field, therefore at high frequency radiation causes the attenuation in the energy provided by the source towards the load.
When the current flow takes place through the transmission line, the conducting wires of the line starts to become heat up. This heating of the lines reduces the energy provided by the source to the load.
Dielectric Loss or Heating
As the transmission lines are composed of two parallel conducting wires and current flow take place through the line. The potential difference exists between the two lines. This potential difference causes the leakage current through dielectric. As a result the heating of dielectric material takes place which reduces the energy provided by source to load.