This document discusses transmission lines and their key characteristics. It begins by defining a transmission line as a medium that directs the transmission of energy, such as electromagnetic waves, from one place to another. Transmission lines are designed to carry alternating current and have a more specific meaning in communications and electronics where their wave nature must be accounted for. The document then discusses some key aspects of transmission line operation, including that voltage and current are functions of both time and position along the line. It provides examples of common transmission line structures and discusses transmission line equivalent circuits and relevant equations for modeling transmission line behavior.

1. Unit-1
Methods of Communication

2. TRANSMISSION LINE
• A transmission line is a material medium or structure that forms a
path for directing the transmission of energy from one place to
another, such as electromagnetic waves or acoustic waves, as well as
electric power transmission.
However in communications and electronic engineering, the term
has a more specific meaning. In these fields, transmission lines are
specialized cables and other media designed to carry
alternating current and electromagnetic waves of radio frequency,
that is, currents with a frequency high enough that its wave nature
must be taken into account. Transmission lines are used for purposes
such as connecting radio transmitters and receivers with their
antennas, distributing cable television signals, and computer
network connections.

3. Transmission Line Concept
Power
Plant
Consumer
Home
Power Frequency (f) is @ 60 Hz
Wavelength (λ) is 5× 106
m
( Over 3,100 Miles)

4. Key point about transmission line operation
The major deviation from circuit theory with
transmission line, distributed networks is this
positional dependence of voltage and current!
– Must think in terms of position and time to
understand transmission line behavior
– This positional dependence is added when the
assumption of the size of the circuit being small
compared to the signaling wavelength
( )
( )tzfI
tzfV
,
,
=
=
V1 V2
dz
I2I1
Voltage and current on a transmission line is
a function of both time and position.

5. Transmission Lines Class 6
Examples of Transmission Line
Structures- I
• Cables and wires
(a)Coax cable
(b) Wire over ground
(c)Tri-lead wire
(d) Twisted pair (two-wire line)
• Long distance interconnects
(a) (b)
(c) (d)
+
-
+
+ +
-
- -
-

6. Segment 2: Transmission line equivalent circuits
and relevant equations
 Physics of transmission line structures
 Basic transmission line equivalent circuit
 ?Equations for transmission line propagation
 Physics of transmission line structures
 Basic transmission line equivalent circuit
 ?Equations for transmission line propagation

7. Remember fields are setup given
an applied forcing function.
(Source)
How does the signal move
from source to load?
E & H Fields – Microstrip Case
The signal is really the wave
propagating between the conductors
Electric field
Magnetic field
Ground return path
X
Y
Z (into the page)
Signal path
Electric field
Magnetic field
Ground return path
X
Y
Z (into the page)
Signal path

8. Transmission Line “Definition”
• General transmission line: a closed system in which power is transmitted
from a source to a destination
• Our class: only TEM mode transmission lines
– A two conductor wire system with the wires in close proximity, providing
relative impedance, velocity and closed current return path to the source.
– Characteristic impedance is the ratio of the voltage and current waves at
any one position on the transmission line
– Propagation velocity is the speed with which signals are transmitted
through the transmission line in its surrounding medium.
I
V
Z =0
r
c
v
ε
=

9. Presence of Electric and Magnetic Fields
• Both Electric and Magnetic fields are present in the transmission lines
– These fields are perpendicular to each other and to the direction of wave
propagation for TEM mode waves, which is the simplest mode, and assumed for
most simulators(except for microstrip lines which assume “quasi-TEM”, which is an
approximated equivalent for transient response calculations).
• Electric field is established by a potential difference between two
conductors.
– Implies equivalent circuit model must contain capacitor.
• Magnetic field induced by current flowing on the line
– Implies equivalent circuit model must contain inductor.
V
I
I
E
+
-
+
-
+
-
+
-
V + ∆V
I + ∆I
I + ∆I
V
I
H
I
H
V + ∆V
I + ∆I
I + ∆I

10. • General Characteristics of Transmission
Line
– Propagation delay per unit length (T0) { time/distance} [ps/in]
• Or Velocity (v0) {distance/ time} [in/ps]
– Characteristic Impedance (Z0)
– Per-unit-length Capacitance (C0) [pf/in]
– Per-unit-length Inductance (L0) [nf/in]
– Per-unit-length (Series) Resistance (R0) [Ω/in]
– Per-unit-length (Parallel) Conductance (G0) [S/in]
T-Line Equivalent Circuit
lL0lR0
lC0
lG0

11. Equations & Formulas
How to model & explain
transmission line behavior

12. Relevant Transmission Line Equations
Propagation equation
βαωωγ jCjGLjR +=++= ))((
)(
)(
0
CjG
LjR
Z
ω
ω
+
+
=
Characteristic Impedance equation
In class problem: Derive the high frequency, lossless
approximation for Z0
α is the attenuation (loss) factor
β is the phase (velocity) factor

13. Refection coefficient
• Signal on a transmission line can be analyzed by keeping track
of and adding reflections and transmissions from the “bumps”
(discontinuities)
• Refection coefficient
– Amount of signal reflected from the “bump”
– Frequency domain ρ=sign(S11)*|S11|
– If at load or source the reflection may be called gamma (ΓL or Γs)
– Time domain ρ is only defined a location
• The “bump”
– Time domain analysis is causal.
– Frequency domain is for all time.
– We use similar terms – be careful
• Reflection diagrams – more later

14. Reflection and Transmission
ρ
1+ρIncident
Reflected
Transmitted
Reflection Coeficient Transmission Coeffiecent
τ 1 ρ+( ) "" ""→ τ 1
Zt Z0−
Zt Z0+
+
ρ
Zt Z0−
Zt Z0+
τ
2 Zt⋅
Zt Z0+

15. • Different
• types of antennas & their characteristics

16. Radio Propagation
• Lower frequencies, especially AM broadcasts in the
mediumwave (sometimes called "medium frequency") and
long wave bands (and other types of radio frequencies
below that), travel efficiently as a surface wave.
• This is because they are more efficiently diffracted by the
figure of the Earth due to their low frequencies.
Ionospheric reflection is taken into consideration as well.
• The ionosphere reflects frequencies in a certain band,
which often changes due to solar conditions. The Earth has
one refractive index and the atmosphere has another, thus
constituting an interface that supports the surface wave
transmission.

17. • Conductivity of the surface affects the propagation of
ground waves, with more conductive surfaces such as water
providing better propagation. [2]
Increasing the conductivity in a
surface results in less dissipation. [3]
The refractive indices are
subject to spatial and temporal changes. Since the ground is not a
perfect electrical conductor, ground waves are attenuated as they
follow the earth’s surface.
• Most long-distance LF "longwave" radio communication (between
30 kHz and 300 kHz) is a result of groundwave propagation.
Mediumwave radio transmissions (frequencies between 300 kHz
and 3000 kHz) have the property of following the curvature of the
earth (the groundwave) in the majority of occurrences. At low
frequencies, ground losses are low and become lower at lower
frequencies. The VLF and LF frequencies are mostly used for
military communications, especially with ships and submarines.

18. • Surface waves have been used in
over-the-horizon radar. In the development of radio,
surface waves were used extensively. Early commercial
and professional radio services relied exclusively on
long wave, low frequencies and ground-wave
propagation.
• To prevent interference with these services, amateur
and experimental transmitters were restricted to the
higher (HF) frequencies, felt to be useless since their
ground-wave range was limited.
• Upon discovery of the other propagation modes
possible at medium wave and short wave frequencies,
the advantages of HF for commercial and military
purposes became apparent. Amateur experimentation
was then confined only to authorized frequencies in
the range.

19. • Medium wave and shortwave reflect off the
ionosphere at night, which is known as skywave.
During daylight hours, the lower "D" layer of the
ionosphere forms and absorbs lower frequency
energy.
• This prevents sky wave propagation from being
very effective on medium wave frequencies in
daylight hours. At night, when the "D" layer
dissipates, medium wave transmissions travel
better by sky wave.
• Ground waves do not include ionospheric and
tropospheric waves.

20. Klystron
•A klystron is a specialized linear-beam
vacuum tube (evacuated electron tube).
• Klystrons are used as amplifiers at
microwave and radio frequencies to
produce both low-power reference signals
for superheterodyne radar receivers and to
produce high-power carrier waves for
communications and the driving force for
modern particle accelerators.

21. Two-cavity klystron amplifier

22. Two-cavity klystron amplifier
• In the two-chamber klystron, the electron beam
is injected into a resonant cavity. The electron
beam, accelerated by a positive potential, is
constrained to travel through a cylindrical drift
tube in a straight path by an axial magnetic field.
While passing through the first cavity, the
electron beam is velocity modulated by the weak
RF signal.
• In the moving frame of the electron beam, the
velocity modulation is equivalent to a
plasma oscillation.

23. • Plasma oscillations are rapid oscillations of the
electron density in conducting media such as plasmas
or metals.(The frequency only depends weakly on the
wavelength). So in a quarter of one period of the
plasma frequency, the velocity modulation is
converted to density modulation, i.e. bunches of
electrons.
• As the bunched electrons enter the second chamber
they induce standing waves at the same frequency as
the input signal. The signal induced in the second
chamber is much stronger than that in the first.

24. Reflex klystron

25. Reflex klystron
• In the reflex klystron (also known as a 'Sutton' klystron after its inventor), the electron beam
passes through a single resonant cavity. The electrons are fired into one end of the tube by
an electron gun. After passing through the resonant cavity they are reflected by a negatively
charged reflector electrode for another pass through the cavity, where they are then
collected
• . The electron beam is velocity modulated when it first passes through the cavity. The
formation of electron bunches takes place in the drift space between the reflector and the
cavity. The voltage on the reflector must be adjusted so that the bunching is at a maximum
as the electron beam re-enters the resonant cavity, thus ensuring a maximum of energy is
transferred from the electron beam to the RF oscillations in the cavity.
• The voltage should always be switched on before providing the input to the reflex klystron
as the whole function of the reflex klystron would be destroyed if the supply is provided after
the input. The reflector voltage may be varied slightly from the optimum value, which results
in some loss of output power, but also in a variation in frequency. This effect is used to good
advantage for automatic frequency control in receivers, and in frequency modulation for
transmitters.
• The level of modulation applied for transmission is small enough that the power output
essentially remains constant. At regions far from the optimum voltage, no oscillations are
obtained at all. This tube is called a reflex klystron because it repels the input supply or
performs the opposite function of a klystron.

26. • There are often several regions of reflector voltage
where the reflex klystron will oscillate; these are
referred to as modes.
• The electronic tuning range of the reflex klystron is
usually referred to as the variation in frequency
between half power points—the points in the
oscillating mode where the power output is half the
maximum output in the mode.
• The frequency of oscillation is dependent on the
reflector voltage, and varying this provides a crude
method of frequency modulating the oscillation
frequency, albeit with accompanying amplitude
modulation as well.
• Modern semiconductor technology has effectively
replaced the reflex klystron in most applications.

27. Magnetron
• All cavity magnetrons consist of a hot cathode with a high (continuous or
pulsed) negative potential by a high-voltage, direct-current power supply.
The cathode is built into the center of an evacuated, lobed, circular
chamber. A magnetic field parallel to the filament is imposed by a
permanent magnet. The magnetic field causes the electrons, attracted to
the (relatively) positive outer part of the chamber, to spiral outward in a
circular path rather than moving directly to this anode. Spaced around the
rim of the chamber are cylindrical cavities. The cavities are open along
their length and connect the common cavity space. As electrons sweep
past these openings, they induce a resonant, high-frequency radio field in
the cavity, which in turn causes the electrons to bunch into groups. A
portion of this field is extracted with a short antenna that is connected to
a waveguide (a metal tube usually of rectangular cross section). The
waveguide directs the extracted RF energy to the load, which may be a
cooking chamber in a microwave oven or a high-gain antenna in the case
of radar.

29. • A cross-sectional diagram of a resonant cavity
magnetron. Magnetic lines of force are parallel to the
geometric axis of this structure.
• The sizes of the cavities determine the resonant
frequency, and thereby the frequency of emitted
microwaves. However, the frequency is not precisely
controllable. The operating frequency varies with
changes in load impedance, with changes in the supply
current, and with the temperature of the tube.[5]
This is
not a problem in uses such as heating, or in some
forms of radar where the receiver can be synchronized
with an imprecise magnetron frequency. Where
precise frequencies are needed, other devices such as
the klystron are used.

30. • The magnetron is a self-oscillating device requiring no external
elements other than a power supply. A well-defined threshold
anode voltage must be applied before oscillation will build up; this
voltage is a function of the dimensions of the resonant cavity, and
the applied magnetic field. In pulsed applications there is a delay of
several cycles before the oscillator achieves full peak power, and
the build-up of anode voltage must be coordinated with the build-
up of oscillator output.[5]
• The magnetron is a fairly efficient device. In a microwave oven, for
instance, a 1.1 kilowatt input will generally create about 700 watt
of microwave power, an efficiency of around 65%. (The high-
voltage and the properties of the cathode determine the power of a
magnetron.) Large S-band magnetrons can produce up to 2.5
megawatts peak power with an average power of 3.75 kW.[5]
Large
magnetrons can be water cooled. The magnetron remains in
widespread use in roles which require high power, but where
precise frequency control is unimportant.