Course Notes for ECE 1266 Applications of fields and waves
NOTES for Transmission Lines I
This lecture covers Chapter 6.1 and 6.2
1. Transmission lines (T-lines), wavelength,
propagation mode
2. Lumped-element model
3. Telegrapher’s equation and solution
1. Transmission lines, wavelength, and TEM wave
Transmission lines are used as a carrier to transmit
transverse EM waves. A T-line connects generator circuits
to loads. A transverse EM (TEM) wave is a special EM
wave, which will be defined below.
When do we need to consider a “line” to be T-line? When
the wavelength of TEM waves is comparable or smaller
than the length of “the line”, we will need to consider the
“line” a T-line. Just give a few rough examples
oA coaxial cable of 1 m length is just a wire if it
transmits signal with frequency of 1 MHz. Because the
wavelength of the TEM wave at this frequency is c/f~
100-300 meter (the speed of the light on the coaxial
cable is smaller than 3 108 meter/s). However when the
signal has frequency of 1 GHz, “this line” becomes a
T-line because the wavelength of the EM wave is <0.3
meter, and therefore the voltage along the line varies
along “the line”.
oConducting wires on a CPU chip can be a few
meters long. They are just “a line” when clock
frequency is ~10 MHz (20 years ago). They must be
considered T-lines today because the CPU runs at a 2-
3 GHz clock frequency.
Below are some examples of T-lines. Not all of the lines
can be considered as T-lines. Strictly speaking, only
structures that support TEM waves can be considered T-
lines. Because the E-field starts from one conducting plate
and ends at the other one, which means the voltages
Chapter 6: Lecture 1 1
