Tran Smiss Io N Li Ne

TRAN SMISS IO N LI NE

Character ist ic s • It is a metallic conductor system used to transfer electrical energy from one point to another using electrical current flow. • Nowadays, the transmission line is made of parallel-conductor (copper wire) and coaxial cable. • It consists of 2-wire line since transmission line for transverse electromagnetic TEM wave propagation always have 2 conductors. • The characteristic of Transmission Line are determined by its electrical properties : Conductivity & Insulator Dielectric Constant September 18, 2009

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Types of Transmission Lines • • • •

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Open-Wire Twin-Lead Twisted-Pair Coaxial

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Metallic transmission lines

Open-wire

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Twin lead

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Metallic transmission lines Unshielded twisted-pair

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Metallic transmission lines Coaxial cable

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Metallic transmission lines Differential, or balanced, transmission system

Balanced lines have equal impedances from the two conductors to ground Twisted-pair and parallel lines are usually balanced

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Ideal Transmission Line • 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

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 The electrical properties determine the PRIMARY electrical constant:  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-wire parallel transmission line and single section transmission line.  The transmission line characteristic is called SECONDARY CONSTANT.  The secondary constants are: 1. Characteristic Impedance 2. Propagation constant September 18, 2009

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Two-wire parallel transmission line electrical equivalent circuit

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Char acteri stic Impedance The characteristic Impedance, Zo is defined as : A transmission line must be terminated at purely resistive load for maximum power transfer.

 Zo =  Zo =  Zo =

( R + jwL) / (G + jwC )

R/G

for Low Frequencies for high Frequencies

L/C

The conductance between 2 wires are determined by the shunt leakage resistance, Rs The load impedance, ZL must match with characteristic impedance, ZO September 18, 2009

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 Refer to figure 12-16 for Characteristic impedance of a transmission line of infinite section.  The characteristic impedance can be calculated by using Ohm’s Law: Zo = Eo / Io where Eo is source voltage and Io is transmission line current

 The characteristic impedance also can be calculated by its physical dimension:

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Any transmission line that is terminated is a purely resistive Load in infinite line:

 Zi = Zo  No Reflected Wave  V & I in phase  Maximum Power Transfer from source to load

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Propagation C

onsta nt

 Propagation Constant is used to express the attenuation (signal loss) and the phase shift per unit length of a transmission line.  It is defined as :

γ = α+ jβ γ = ( R + jwL)(G + jwC ) Where γ = propagation constant (unitless) α = attenuation coefficient (nepers per unit length) β = phase shift coefficient (radians per unit length) September 18, 2009

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For an ideal line R and G are zero

  j LC  0

purely imaginary and no attenuation

   LC

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Transmission Lines Losses • Conductor Losses •Increases with frequency due to skin effect

• Dielectric Heating Losses •Also increases with frequency • Radiation

Losses

• Not significant with good quality coax properly installed • Can be a problem with openwire cable • Coupling

Losses

• Corona Skin effect September 18, 2009

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Transmission Lines Losses

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