19911515 Transmission Lines1

8/8/2019 19911515 Transmission Lines1

1/17

TRANSMISSION LINE


2/17

18 November 2010 FKEE 2

Characteristics

It is a metallic conductor system used to transfer

electrical energy from one point to another usingelectrical current flow.

Nowadays, the transmission line is made ofparallel-conductor (copper wire) and coaxial

cable.

It consists of 2-wire line since transmission linefor transverse electromagnetic TEM wave

propagation always have 2 conductors.

The characteristic of Transmission Line aredetermined by its electrical properties :Conductivity & Insulator Dielectric Constant


3/17


Typesof TransmissionLines

Open-Wire

Twin-Lead

Twisted-Pair

Coaxial


4/17


Metallic transmission lines

Open-wire Twin lead


5/17



Unshielded twisted-pair


6/17



Coaxial cable


7/17



Balanced lines have equal impedances from the two

conductors to ground

Twisted-pair andparallel lines are usually balanced

Differential, or balanced, transmission system


8/17


IdealTransmissionLine

No losses

conductors have zero resistance

dielectric has zero conductance

possible only with superconductors

approximated by a short line

No capacitance or inductance

possible with a real line only at dc

with low frequencies and short lines this can

be approximated


9/17


The electrical properties determine the PRIMARY electricalconstant:

Series Resistance, R

Series Inductance, L

Shunt Capacitance, C

Shunt Conductance, G

The combined above parameter is called LUMPED

PARAMETERS.

Refer to Figure 12-14 & 12-15 in the textbook for the two-wireparallel transmission line and single section transmissionline.

The transmission line characteristic is called SECONDARYCONSTANT.

The secondary constants are:

1. Characteristic Impedance

2. Propagation constant


10/17


Two-wire parallel transmission line

electrical equivalent circuit


11/17


The characteristic Impedance, Zo is defined as :

A transmission line must be terminated at purelyresistive load for maximum power transfer.

Zo =

Zo = for Low Frequencies

Zo = for high Frequencies

The conductance between 2 wires are determinedby the shunt leakage resistance, Rs

The load impedance, ZL must match withcharacteristic impedance, ZO

GR /

)(/)( jwCGjwLR

CL /

CharacteristicImpedance


12/17


Refer to figure 12-16 for Characteristic impedance

of a transmission line of infinite section.

The characteristic impedance can be calculated

by usingOhms Law:

Zo = Eo/ Io

where Eo is source voltage andIo is transmission linecurrent

The characteristic impedance also can becalculated by its physical dimension:


13/17


Any transmission line that is terminated is apurely resistive Load in infinite line:

Zi= Zo

No Reflected Wave

V & I in phase

Maximum Power Transfer from source to load


14/17


Propagation Constant is used to express the

attenuation (signal loss) and the phase shift perunit length of a transmission line.

It is defined as :

Where K = propagation constant (unitless) = attenuation coefficient (nepers per unit

length)

= phase shift coefficient (radians per unit

length)

))(( jwCGjwLR !K

PropagationConstant

K = + j


15/17


16/17


Transmission Lines Losses

Conductor LossesIncreases with frequency due

to skin effect

Dielectric Heating LossesAlso increases with frequency

Radiation Losses Not significant with good

quality coaxproperly installed

Can be a problem with open-

wire cable

Coupling Losses

Corona

Skin effect


17/17


Transmission Lines Losses

19911515 Transmission Lines1

Documents

Transcript of 19911515 Transmission Lines1