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Transmission media part2 K. MEENAKSHI 


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Characteristics of Transmission lines Characteristics of transmission line depends on:
Electrical properties:
Conductor length, spacing b/w dielectric material. This in turn determine primary electrical constants. They are
1) series resistance
2) series inductance.
3) Shunt capacitance.
4)Shunt conductance.


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Characteristic impedance It is defined as impedance seen looking into infinitely long line or impedance seen looking into finite length terminated by purely resistive load with resistance equal to the characteristic impedance of the line.
The characteristic impedance is simply the ratio b/w source voltage (E0 ) to the line current(I0 ).Mathematically
Z0 = 

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Continued…. Characteristic impedance of two wire parallel line with air as dielectric:
Z0 = 276 log
Where Z0 =characteristic impedance
D= distance b/w the centers of two conductors.
R= radius of the conductor.
problem:
Determine the characteristic impedance of two wire parallel transmission line with D/r ratio of 12.22
Z0 =276 log(12.22)=300 ?


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continued Where Z0 =characteristic impedance
D= distance b/w the centers of two conductors.
R= radius of the conductor.
Determine the characteristic impedence of two wire parallel transmission line with D/r ratio of 12.22
Z0 =276 log(12.22)=300 ? 

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Characteristic impudence of coaxial cable
Z0 =
Where Z0 = Characteristic impedance.
D=input diameter of inner conductor.
=relative permittivity of dielectric constant.
problem: Determine the characteristic impedance of the RG59A with the following specifications. D=0.025 inches , D=0.15 in and =2.23
Z0 = log
=71.9?


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Velocity Factor Velocity Factor is defined as the ratio of the actual velocity of electromagnetic wave through the given medium to the velocity of propagation through the vacuum.
Vf = velocity factor
Vp =actual velocity of propagation
c=velocity of propagation through vacuum. 

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Cont… Vf *c= Vp
The velocity of the propagation depends on the dielectric constant of the insulating material separating the two conductors.
Vp =
where is the dielectric constant of the material.
It is normally given in percentage.


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Dielectric constant It is simply relative permittivity of the material. The relative dielectric constant of air is 1.006.
The velocity of the electromagnetic wave varies with inductance and capacitance of the material.
so T=
Vp = 

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problem Problem: For a given RG 8A/U coaxial cable with the distributed inductance L=241.56 Nh/m and c=96.6 Pf/m and dielectric constant is
2.3 . Determine the velocity of propagation and velocity factor?
Vp=
= m/s
and velocity factor= =0.66 

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Losses in Transmission lines Different types of losses in Transmission line
Conductor losses
Dielectric heating losses
Radiation losses
Coupling losses
corona


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Skin effect All electric currents generate magnetic fields that in turn can affect the current (this is the principle behind electric guitar pickups). In a direct current case everything is constant and so nothing seems to happen. With an alternating current, however, there is a delay in the magnetic field's response to the change in current and the 'old' magnetic field tends to push the current towards the outside of the conductor. As the frequency increases, so does the effect until at very high frequencies the entire current flows in a very narrow skin on the conductorhence the name. 

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Conductor losses At high frequencies most of the current flows along the surface(outer skin) and increasing the opposition to the current in side the conductor. This is 00 phase and equivalent to the resistance. The loss due to this resistance is called conductor loss. 

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Other losses Dielectric heating loss: For air dielectric transmission this is negligible. For solid dielectric materials dielectric losses increases with the frequency.
Radiation losses: If the separation b/w the conductors is an appreciable fraction of the wave length , the electrostatic and electromagnetic fields that surrounds the conductor cause the line to act as antenna and transfer energy to the nearby conductive material and the loss is termed radiation loss.


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Contd…. Coupling losses: occur when two sections of transmission lines meet , causing mechanical discontinuities that heat up and dissipate power in the form of coupling loss.
Corona:Corona is a luminous discharge that occur b/w two conductors of a transmission line when the difference of the potential exceeds the break down voltage. It destroys the transmission lines. 
