TRANSMISSION LINE (TL) THEORY

WEEK 1

• CLASS 1: The lumped-element circuit model for a transmission line, Wave propagation in a transmission line

• CLASS 2: Field analysis of transmission lines, Classification of transmission lines,

WEEK 2

• CLASS 3: The terminated lossless transmission line, VSWR, The Smith Chart.

• CLASS 4: The quarter wave transformer, Transients on transmission lines, Generator and load mismatches

MICROWAVE TRANSMISSION LINES

A transmission line is a system that is capable of “guiding” the energy transfer between a generator and a load

Derivation of the Telegrapher’s Eqtn Equation (6) - Board

} Gives rise to voltage and current that is a function of time and

distance

Lumped element circuit model of

a TL

WAVE PROPAGATION IN A TRANSMISSION LINE

Wave Propagation in TL Equation: Equation (7) - Board

TYPES OF TRANSMISSION LINES

Microstrip Line Waveguide Coax Cable

Field Lines

SEVERAL “CONCERNS” WITH TL

CHARACTERISTIC IMPEDANCE

CUT-OFF FREQUENCY

ELECTROMAGNETIC MODE

Ratio of the voltage to current when

there is no reflection from the load

(infinitely long line)

Frequency at which the desirable field

patterns are excited in any type of TL.

The EM field pattern measured at a plane perpendicular (i.e., transverse) to the

propagation direction of the wave. Note that

a TL can have more than one mode

TYPE OF EM MODE

Transverse Electric-Magnetic (TEM)

Transverse Electric(TE)

Transverse Magnetic(TM)

Neither electric nor magnetic field in the

direction of propagation

No electric field in the direction of propagation.

These are sometimes called H modes because there is only a magnetic field along the direction

of propagation

No magnetic field in the direction of propagation.

These are sometimes called E modes because there is only an electric

field along the direction of propagation

MICROSTRIP LINE

Dielectric

CHARACTERISTIC IMPEDANCE

CUT-OFF FREQUENCY

ELECTROMAGNETIC MODE

SIDE VIEW

Fundamental Mode:Quasi TEM

@ ‘d’

Anything above the fc, a different EM mode is excited (higher-order mode)

RECTANGULAR WAVEGUIDE

Dielectric

CHARACTERISTIC IMPEDANCE

CUT-OFF FREQUENCY

ELECTROMAGNETIC MODE

Fundamental Mode:TE(10)

@ ‘d’

Anything above the fc, a different EM mode is excited (higher-order mode)

TE TM