Communications 2 EE555

1
Communications 2EE555
2
Course Content

• Introduction Review
• Transmission Line Characteristics
• Waveguides Microwave Devices
• Radiowave Propagation
• Antennas
• Microwave Radio Radar Systems
• Fibre Optic Communications

3
Introduction Review

• Microwaves are defined as radio waves in the
  frequency range gt 1 GHz.
• However, waves gt 20 GHz are commonly known as
  millimeter waves
• Distributed, rather than lumped, circuit elements
  must be used at microwave frequencies because of
  a phenomenon called skin effect.

4
Skin Effect

• At microwave frequencies current travels on the
  outer surface, or skin, of the conductor because
  of the increased inductance created.
• The skin depth , ? (in m), for a conductor with
  permeability, ? (in H/m), conductivity, ? (in
  S/m), and at a frequency, f (in Hz), is given by

5
Skin Effect (contd)
J

• The current density, J, decreases with the
  distance beneath the surface exponentially.
• At a depth ?, the current density decreases to
  Jo/e.
• As f increases, ? ? and resistance ?.

Jo
J Joe-z/d
z
conductor surface
direction of current
6
Transverse Electromagnetic Waves
In free space
z
Direction of Propagation
y
Magnetic Field
Electric Field
x
7
Notes on TEM Waves

• The E- and H-fields and the direction of motion
  of TEM waves are mutually perpendicular to each
  other.
• Velocity of radio waves in free space is c
  3x108 m/s, but in a medium with dielectric
  constant ?r

8
Microwave Materials

• Glass epoxy printed circuit boards are unsuitable
  for microwave use because of high dissipation
  factor and wide tolerance in thickness and
  dielectric constant.
• Instead, materials such as Teflon fiberglass
  laminates, alumina substrates, sapphire and
  quartz substrates must be used (refer to text for
  details).

9
Types of Transmission Lines

• Differential or balanced lines (where neither
  conductor is grounded) e.g. twin lead,
  twisted-cable pair, and shielded-cable pair.
• Single-ended or unbalanced lines (where one
  conductor is grounded) e.g. concentric or
  coaxial cable.
• Transmission lines for microwave use e.g.
  striplines, microstrips, and waveguides.

10
Transmission Line Equivalent Circuit
L
L
L
L
R
R
Zo
Zo
C
C
C
C
G
G
Lossless Line
Lossy Line
11
Notes on Transmission Line

• Characteristics of a line is determined by its
  primary electrical constants or distributed
  parameters R (?/m), L (H/m), C (F/m), and G
  (S/m).
• Characteristic impedance, Zo, is defined as the
  input impedance of an infinite line or that of a
  finite line terminated with a load impedance, ZL
  Zo.

12
Formulas for Some Lines
For parallel two-wire line
D
d
m momr e eoer mo 4px10-7 H/m eo 8.854
pF/m
For co-axial cable
D
d
13
Transmission-Line Wave Propagation
Electromagnetic waves travel at lt c in a
transmission line because of the dielectric
separating the conductors. The velocity of
propagation is given by
m/s
Velocity factor, VF, is defined as
14
Propagation Constant

• Propagation constant, ?, determines the variation
  of V or I with distance along the line V
  Vse-x? I Ise-x?, where VS, and IS are the
  voltage and current at the source end, and x
  distance from source.
• ? ? j?, where ? attenuation coefficient (
  0 for lossless line), and ? phase shift
  coefficient 2?/? (rad./m)

15
Incident Reflected Waves

• For an infinitely long line or a line terminated
  with a matched load, no incident power is
  reflected. The line is called a flat or
  nonresonant line.
• For a finite line with no matching termination,
  part or all of the incident voltage and current
  will be reflected.

16
Reflection Coefficient
The reflection coefficient is defined as
It can also be shown that
Note that when ZL Zo, ? 0 when ZL 0, ?
-1 and when ZL open circuit, ? 1.
17
Standing Waves
Vmax Ei Er
Voltage
Vmin Ei - Er
l 2
With a mismatched line, the incident and
reflected waves set up an interference pattern on
the line known as a standing wave. The standing
wave ratio is
18
Other Formulas
When the load is purely resistive (whichever
gives an SWR gt 1)
Return Loss, RL Fraction of power reflected
?2, or -20 log ? dB So, Pr ?2Pi
Mismatched Loss, ML Fraction of
power transmitted/absorbed 1 - ?2 or -10
log(1-?2) dB So, Pt Pi (1 - ?2) Pi - Pr
19
Time-Domain Reflectometry
d
ZL
Transmission Line
Oscilloscope
Pulse or Step Generator
TDR is a practical technique for determining
the length of the line, the way it is terminated,
and the type and location of any impedance
discontinuities. The distance to the
discontinuity is d vt/2, where t elapsed
time of returned reflection.
20
Typical TDR Waveform Displays
Vr
Vr
Vi
Vi
t
RL lt Zo
RL gt Zo
ZL capacitive
ZL inductive
21
Transmission-Line Input Impedance
The input impedance at a distance l from the load
is
When the load is a short circuit, Zi jZo tan
(?l).
For 0 ? l lt ?/4, shorted line is inductive.
For l ?/4, shorted line a parallel resonant
circuit.
For ?/4 lt l ? ?/2, shorted line is capacitive.
22
T-L Input Impedance (contd)

• When the load is an open circuit, Zi -jZo cot
  (?l)
• For 0 lt l lt ?/4, open circuited line is
  capacitive.
• For l ?/4, open-line series resonant circuit.
• For ?/4 lt l lt ?/2, open-line is inductive.
• A ?/4 line with characteristic impedance, Zo,
  can be used as a matching transformer between a
  resistive load, ZL, and a line with
  characteristic impedance, Zo, by choosing

23
Transmission Line Summary
or
is equivalent to
l gt ?/4
l lt ?/4
is equivalent to
or
l gt ?/4
l lt ?/4
?/4
Zo
ZL

Zo
l ?/4
?/4-section Matching Transformer

24
The Smith Chart

• The Smith chart is a graphical aid to solving
  transmission-line impedance problems.
• The coordinates on the chart are based on the
  intersection of two sets of orthogonal circles.
• One set represents the normalized resistive
  component, r ( R/Zo), and the other the
  normalized reactive component, jx ( jX/Zo).

25
Smith Chart Basics
j0.7
r 0
z1 1j0.7
z1
r 2
j0
?
z2
z2 2-j1.4
r 1
-j1.4
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Applications of The Smith Chart

• Applications to be discussed in this course
• Find SWR, ???, RL
• Find YL
• Find Zi of a shorted or open line of length l
• Find Zi of a line terminated with ZL
• Find distance to Vmax and Vmin from ZL
• Solution for quarter-wave transformer matching
• Solution for parallel single-stub matching

27
Substrate Lines

• Miniaturized microwave circuits use striplines
  and microstrips rather than coaxial cables as
  transmission lines for greater flexibility and
  compactness in design.
• The basic stripline structure consists of a flat
  conductor embedded in a dielectric material and
  sandwiched between two ground planes.

28
Basic Stripline Structure
Ground Planes
W
b
t
er
Solid Dielectric
Centre Conductor
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Notes On Striplines

• When properly designed, the E and H fields of the
  signal are completely confined within the
  dielectric material between the two ground
  planes.
• The characteristic impedance of the stripline is
  a function of its line geometry, specifically,
  the t/b and w/b ratios, and the dielectric
  constant, ?r.
• Graphs, design formulas, or computer programs are
  available to determine w for a desired Zo, t, and
  b.

30
Microstrip
w
Circuit Line
t
b
?r (dielectric)
Ground Plane
Microstrip line employs a single ground plane,
the conductor pattern on the top surface being
open.
Graphs, formulas or computer programs would be
used to design the conductor line width.
However, since the electromagnetic field is
partly in the solid dielectric, and partly in the
air space, the effective relative permittivity,
?eff, has to be used in the design instead of ?r.
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Stripline vs Microstrip

• Advantages of stripline
• signal is shielded from external interference
• shielding prevents radiation loss
• ?r and mode of propagation are more predictable
  for design
• Advantages of microstrip
• easier to fabricate, therefore less costly
• easier to lay, repair/replace components

32
Microstrip Directional Coupler
2
4
Conductor Lines
?/4
Dielectric
Ground Plane
Top View
Cross-sectional View
1
3
Most of the power into port 1 will flow to port
3. Some of the power will be coupled to port 2
but only a minute amount will go to port 4.
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Formulas For Directional Coupler
The operation of the coupler gives rise to an
even mode characteristic impedance, Zoe, and an
odd mode characteristic impedance, Zoo, where
For a given coupling factor, C (which is V2/V1)
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Coupler Applications

• Some common applications for couplers
• monitoring/measuring the power or frequency at a
  point in the circuit
• sampling the microwave energy for used in
  automatic leveling circuits (ALC)
• reflection measurements which indirectly yield
  information on VSWR, ZL, return loss, etc.

35
Branch Coupler
Z1 0.707 Zo Input power at port 1 will divide
equally between Ports 2 and 3 and none to port
4.
l/4
4
3
Z1
l/4
Zo
Zo
Z1
2
1
Can provide tighter coupling and can
handle higher power than directional coupler.
Branches may consist of chokes, filters, or
matched load for more design flexibility.
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Hybrid Ring Coupler
Input power at port 1 divides evenly between
ports 2 4 and none for port 3.
3l/4
4
1
l/4
l/4
Similarly, input at port 2 will divide evenly
between ports 1 and 3 and none for port 4.
l/4
3
2
One application circulator.
37
Microstrip Stripline Filters
?/4
IN
OUT
Side-coupled half-wave resonator band-pass filter
IN
OUT
L
L
L
L
C
C
C
Conventional low-pass filter
38
Scattering Parameters

• Microwave devices are often characterized by
  their S-parameters because
• measurement of V and I may be difficult at
  microwave frequencies.
• Active devices frequently become unstable when
  open or short-circuit type measurements are made
  for h, Y or Z parameters.
• An S matrix is used to contain all the
  S-parameters.

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S-Variables S-Parameters
a1
a2
2-Port Network
V1
V2
b1
b2
For port x Vx Vix Vrx
S-variables
Px Pix - Prx ax2-bx2
b1 S11a1 S12a2 b2 S21a1 S22a2
or
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S-Parameters of 2-Port Network
Note when port 2 is terminated with a matched
load, a2 0. Similarly, a1 0 when port 1 is
matched.
S11, and S22 are reflection coefficients, i.e.,
?11, ?22.
S21 represents the forward transmission
coefficient. Thus, Insertion Loss/attenuation
-10 log (Po2/Pi1) -20 log S21 dB
S12 is the reversed transmission coefficient.
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Properties of S-Parameters

• In general, S-parameters have both magnitude and
  angle.
• For matched 2-port reflectionless networks, S11
  S22 0
• For a reciprocal 2-port network, S12 S21.
• For a lossless 2-port network, S12 S21 1.
• For n-port, b S a. The n x n S matrix
  characterizes the network.

42
Microwave Radiation Hazards

• The fact that microwaves can be used for cooking
  purposes and in heating applications suggests
  that they have the potential for causing
  biological damage.
• Health Welfare, Canada specifies no limit
  exposure duration for radiation level of 1 mW/cm2
  or less for frequencies from 10 MHz to 300 GHz.
• Avoid being in the direct path of a microwave
  beam coming out of an antenna or waveguide.

43
Waveguides

• Reasons for using waveguide rather than coaxial
  cable at microwave frequency
• easier to fabricate
• no solid dielectric and I2R losses
• Waveguides do not support TEM waves inside
  because of boundary conditions.
• Waves travel zig-zag down the waveguide by
  bouncing from one side wall to the other.

44
E-Field Pattern of TE1 0 Mode
b
a
?g/2
End View
Side View
TEmn means there are m number of half-wave
variations of the transverse E-field along the
a side and n number of half-wave variations
along the b side.
The magnetic field (not shown) forms closed loops
horizontally around the E-field
45
TE and TM Modes

• TEmn mode has the E-field entirely transverse,
  i.e. perpendicular, to the direction of
  propagation.
• TMmn mode has the H-field entirely transverse to
  the direction of propagation.
• All TEmn and TMmn modes are theoretically
  permissible except, in a rectangular waveguide,
  TMmo or TMon modes are not possible since the
  magnetic field must form a closed loop.
• In practice, only the dominant mode, TE10 is
  used.

46
Wavelength for TE TM Modes
Cutoff wavelength

• Any signal with l ? lc will not propagate down
• the waveguide.
• For air-filled waveguide, cutoff freq., fc c/lc

• TE10 is called the dominant mode since lc 2a
• is the longest wavelength of any mode.

Guide wavelength
47
Other Formulas for TE TM Modes
Group velocity
Phase velocity
Wave impedance
Zo 377 W for air-filled waveguide
48
Circular/Cylindrical Waveguides

• Differences versus rectangular waveguides
• lc 2pr/Bmn where r waveguide radius, and Bmn
  is obtained from table of Bessel functions.
• All TEmn and TMmn modes are supported since m and
  n subscripts are defined differently.
• Dominant mode is TE11.
• Advantages higher power-handling capacity, lower
  attenuation for a given cutoff wavelength.
• Disadvantages larger and heavier.

49
Waveguide Terminations
lg/2
Dissipative Vane
Short-circuit
Sliding Short-Circuit
Side View
End View
Dissipative vane is coated with a thin film of
metal which in turn has a thin dielectric coating
for protection. Its impedance is made equal to
the wave impedance. The taper minimizes
reflection.
Sliding short-circuit functions like a shorted
stub for impedance matching purpose.
50
Attenuators
Resistive Flap
Max. attenuation when flap is fully inside. Slot
for flap is chosen to be at a non- radiating
position.
Pi
Po
Rotary-vane Type
Atten.(dB) 10 log (Pi/Po) -20 log S21
Max. attenuation when vane is at centre of guide
and min. at the side-wall.
Pi
Po
Sliding-vane Type
51
Iris Reactors
Inductive iris vanes are vertical

Capacitive iris vanes are horizontal

Irises can be used as reactance elements, filters
or impedance matching devices.

52
Tuning Screw s
Tuning Screws
Post
A post or screw can also serve as a reactive
element. When the screw is advanced partway into
the wave- guide, it acts capacitive. When the
screw is advanced all the way into the waveguide,
it acts inductive. In between the two positions,
one can get a resonant LC circuit.
53
Waveguide T-Junctions
2
3
3
1
2
1
E-Plane Junction
H-Plane Junction
Input power at port 2 will split equally between
ports 1 and 3 but the outputs will be antiphase
for E-plane T and inphase for H-plane T. Input
power at ports 1 3 will combine and exit from
port 1 provided the correct phasing is used.
54
S-Matrix for T-Junctions
For ideal T-junction
Note sign is used for H-plane T, and (-)
sign for E-plane T.
Also note that even though S22 0 (i.e.
lossless), S11 and S33 are each equal to 1/2,
i.e., input power applied to ports 1 and 3 will
always suffer from reflection.
55
Hybrid-T Junction
3
Under matched ideal conditions
2
4
1
It combines E-plane and H-plane junctions.
Note S11, S22, S33, and S44 are zero.
Pin at port 1 or 2 will divide between ports 3
and 4. Pin at port 3 or 4 will divide between
ports 1 and 2.
56
Hybrid-T Junction (contd)

• If input power of the same phase is applied
  simultaneously at ports 1 and 2, the combined
  power exits from port 4. If the input is
  out-of-phase, the output is at port 3.
• Applications
• Combining power from two transmitters.
• TX and a RX sharing a common antenna.
• Low noise mixer circuit.

57
Directional Coupler
lg/4
P4
Termination
P3
P2
P1
P1
P2
2-hole Coupler
Holes spaced lg/4 allow waves travelling
toward port 4 to combine. Waves travelling
toward port 3, however, will cancel. Therefore,
ideally P3 0.
To broaden frequency response bandwidth,
practical couplers would usually have multi holes.
58
Directional Coupler (contd)
For ideal directional coupler
where a2 b2 1
Definitions
Coupling Factor,
Directivity,
Insertion Loss (dB) 10 log (P1/P2) -20 log
S12
59
Cavity Resonators
Resonant wavelength for a rectangular cavity
b
L
a
For a cylindrical resonator
r
L
60
Cavity Resonators (contd)

• Energy is coupled into the cavity either through
  a small opening, by a coupling loop or a coupling
  probe. These methods of coupling also apply for
  waveguides
• Applications of resonators
• filters
• absorption wavemeters
• microwave tubes

61
Ferrite Components

• Ferrites are compounds of metallic oxides such as
  those of Fe, Zn, Mn, Mg, Co, Al, and Ni.
• They have magnetic properties similar to
  ferromagnetic metals and at the same time have
  high resistivity associated with dielectrics.
• Their magnetic properties can be controlled by
  means of an external magnetic field.
• They can be transparent, reflective, absorptive,
  or cause wave rotation depending on the H-field..

62
Examples of Ferrite Devices
Isolator
Attenuator
2
q
3
1
Differential Phase Shifter
4-port Circulator
4
63
Notes On Ferrite Devices

• Differential phase shifter - q is the phase shift
  between the two directions of propagation.
• Isolator - permits power flow in one direction
  only.
• Circulator - power entering port 1 will go to
  port 2 only power entering port 2 will go to
  port 3 only etc.
• Most of the above are based on Faraday rotation.
• Other usage filters, resonators, and substrates.

64
Schottky Barrier Diode
Metal Electrode
Its based on a simple metal- semiconductor
interface. There is no p-n junction but a
depletion region exists. Current is by
majority carriers therefore, very low in
capacitance.
Contact
Semi- conductor Layer
SiO2 Dielectric
Substrate
Metal Electrode
Applications detectors, mixers, and switches.
65
Varactor Diode
Cj
Co
Circuit Symbol
V
Junction Capacitance Characteristic
Varactors operate under reverse-bias
conditions. The junction capacitance is
where Vb barrier potential (0.55 to 0.7 for
silicon) and K constant (often 1)
66
Equivalent Circuit for Varactor
The series resistance, Rs, and diode capacitance,
Cj, determine the cutoff frequency
Cj
Rj
Rs
The diode quality factor for a given frequency,
f, is
67
Varactor Applications

• Voltage-controlled oscillator (VCO) in AFC and
  PLL circuits
• Variable phase shifter
• Harmonic generator in frequency multiplier
  circuits
• Up or down converter circuits
• Parametric amplifier circuits - low noise

68
Parametric Amplifier Circuit
Degenerative Mode fp 2fs
Pump signal (fp)
Nondegenerative mode
L2
Upconversion - fi fs fp Downconversion - fi
fs - fp Power gain, G fi /fs
C2
C1
Input signal (fs)

• Regenerative mode
• negative resistance
• very low noise
• very high gain
• fp fs fi

C3
L3
D1
L1
Idler tank (fi)
Signal tank (fs)
69
PIN Diode
S1
RFC
R
V
P
C2
C1
I
In
Out
N
D1
PIN as shunt switch
PIN diode has an intrinsic region between the
P and N materials. It has a very high
resistance in the OFF mode and a very low
resistance when forward biased.
70
PIN Diode Applications

• To switch devices such as attenuators, filters,
  and amplifiers in and out of the circuit.
• Voltage-variable attenuator
• Amplitude modulator
• Transmit-receive (TR) switch
• Phase shifter (with section of transmission line)

71
Tunnel Diode
i
Ls
Ip
B
Cj
-R
A
C
Rs
V
Equivalent Circuit
Symbol
Vv
Vp
Characteristic Curve
Heavy doping of the semiconductor material
creates a very thin potential barrier in the
depletion zone which leads to electron tunneling
through the barrier. Note the negative resistance
zone between Vp and Vv.
72
More Notes On Tunnel Diode
Tunnel diodes can be used in monostable (A or
C), bistable (between A and C), or astable (B)
modes. These modes lead to switching,
oscillation, and amplification applications.
However, the power output levels of the tunnel
diode are restricted to a few mW only.
The resistive, and self-resonant frequencies are
73
Transferred Electron Devices

• TEDs are made of compound semiconductors such as
  GaAs.
• They exhibit periodic fluctuations of current due
  to negative resistance effects when a threshold
  voltage (about 3.4 V) is exceeded.
• The negative resistance effect is due to
  electrons being swept from a lower valley (more
  mobile) region to an upper valley (less mobile)
  region in the conduction band.

74
Gunn Diode
The Gunn diode is a transferred electron device
that can be used in microwave oscillators or
one-port reflection amplifiers. Its basic
structure is shown below. N-, the active region,
is sandwiched between two heavily doped N
regions. Electrons from the
cathode (K) drifts to the anode (A) in
bunched formation called domains. Note that there
is no p-n junction.
l
N-
A
K
Metallic Electrode
Metallic Electrode
N
75
Gunn Operating Modes

• Stable Amplification (SA) Mode diode behaves as
  an amplifier due to negative resistance effect.
• Transit Time (TT) Mode operating frequency, fo
  vd / l where vd is the domain velocity, and l is
  the effective length. Output power lt 2 W, and
  frequency is between 1 GHz to 18 GHz.
• Limited Space-Charge (LSA) Mode requires a
  high-Q resonant cavity operating frequency up to
  100 GHz and pulsed output power gt 100 W.

76
Gunn Diode Circuit and Applications
Tuning Screw
The resonant cavity is shocked excited by current
pulses from the Gunn diode and the RF energy
is coupled via the iris to the waveguide.
Resonant Cavity
Iris
Diode
V
Gunn diode applications microwave source
for receiver local oscillator, police radars,
and microwave communication links.
77
Avalanche Transit-Time Devices

• If the reverse-bias potential exceeds a certain
  threshold, the diode breaks down.
• Energetic carriers collide with bound electrons
  to create more hole-electron pairs.
• This multiplies to cause a rapid increase in
  reverse current.
• The onset of avalanche current and its drift
  across the diode is out of phase with the applied
  voltage thus producing a negative resistance
  phenomenon.

78
IMPATT Diode
A single-drift structure of an IMPATT
(impact avalanche transit time) diode is shown
below
-

P
N
N
l
Avalanche Region
Drift Region
Operating frequency
where vd drift velocity
79
Notes On IMPATT Diode

• The current build-up and the transit time for the
  current pulse to cross the drift region cause a
  180o phase delay between V and I thus, negative
  R.
• IMPATT diodes typically operate in the 3 to 6 GHz
  region but higher frequencies are possible.
• They must operate in conjunction with an external
  high-Q resonant circuit.
• They have relatively high output power (gt100 W
  pulsed) but are very noisy and not very
  efficient.

80
Microwave Transistors

• Silicon BJTs and GaAsFETs are most widely used.
• BJT useful for amplification up to about 6 MHz.
• MesFET (metal semiconductor FET) and HEMT (high
  electron mobility transistor) are operable beyond
  60 GHz.
• FETs have higher input impedance, better
  efficiency and more frequency stable than BJTs.

81
Microwave Transistor Power Gain
Matching Network GL
Zs
Matching Network Gs
Transistor Go
ZL
Vs
Max. power gain of a unilateral transistor
amplifier with conjugate matched input and output
Note that Go S212 is the gain of the
transistor. For unconditional stability, S11
lt 1 and S22 lt 1.
82
Noise Factor Noise Figure
Noise Factor, Fn SNRin/SNRout
Noise Figure, NF (dB) 10 log Fn
SNRin (dB) - SNRout (dB)
Equivalent noise temperature, Te (Fn -1)
To where To 290 oK
For amplifiers in cascade, the overall noise
factor
where Gn amplifier gain of the nth stage.
83
Microwave Tubes

• Classical vacuum tubes have several factors which
  limit their upper operating frequency
• interelectrode capacitance lead inductance
• dielectric losses skin effect
• transit time
• Microwave tubes utilize resonant cavities and the
  interaction between the electric field, magnetic
  field and the electrons.

84
Magnetrons
It consists of a cylindrical cathode surrounded
by the anode with a number of resonant cavities.
Its a crossed-field device since the E-field is
perpendicular to the dc magnetic field.
Interaction Space
Waveguide Output
At a critical voltage the electrons from the
cathode will just graze the anode.
Cavity
Coupling Window
Anode
Cathode
85
Magnetron Operation

• When an electron cloud sweeps past a cavity, it
  excites the latter to self oscillation which in
  turn causes the electrons to bunch up into a
  spoked wheel formation in the interaction space.
• The continuous exchange of energy between the
  electrons and the cavities sustains oscillations
  at microwave frequency.
• Electrons will eventually lose their energy and
  fall back into the cathode while new ones are
  emitted.

86
More Notes On Magnetrons

• Alternate cavities are strapped (i.e., shorted)
  so that adjacent resonators are 180o out of
  phase. This enables only the dominant p-mode to
  operate.
• Frequency tuning is possible either mechanically
  (screw tuner) or electrically with voltage.
• Magnetrons are used as oscillators for radars,
  beacons, microwave ovens, etc.
• Peak output power is from a few MW at UHF and
  X-band to 10 kW at 100 GHz.

87
Klystrons

• Klystrons are linear-beam devices since the
  E-field is parallel to the static magnetic field.
• Their operation is based on velocity and density
  modulation with resonating cavities to create the
  bunching effect.
• They can be employed as oscillators or power
  amplifiers.

88
Two-Cavity Klystron
RF In
RF Out
Control Grid
Gap
Filament
Collector
Drift Region
Cathode
Buncher Cavity
Catcher Cavity
Anode
v
Electron Beam
Effect of velocity modulation
89
Klystron Operation

• RF signal applied to the buncher cavity sets up
  an alternating field across the buncher gap.
• This field alternately accelerates and
  decelerates the electron beam causing electrons
  to bunch up in the drift region.
• When the electron bundles pass the catcher gap,
  they excite the catcher cavity into resonance.
• RF power is extracted from the catcher cavity by
  the coupling loop.

90
Multicavity Klystrons

• Gain can be increased by inserting intermediate
  cavities between the buncher and catcher cavity.
• Each additional cavity increases power gain by
  15- to 20-dB.
• Synchronous tuned klystrons have high gain but
  very narrow bandwidth, e.g. 0.25 of fo.
• Stagger tuned klystrons have wider bandwidth at
  the expense of gain.
• Can operate as oscillator by positive feedback.

91
Reflex Klystron
Output
Anode
Cavity
Cathode
Repeller
Filament
Electron Beam
Vr
Condition for oscillation requires electron
transit time to be
where n an integer and T period of oscillation
92
Reflex Klystron Operation

• Electron beam is velocity modulated when passing
  though gridded gap of the cavity.
• Repeller decelerates and turns back electrons
  thus causing bunching.
• Electrons are collected on the cavity walls and
  output power can be extracted.
• Repeller voltage, Vr, can be used to vary output
  frequency and power.

93
Notes On Reflex Klystrons

• Only one cavity used.
• No external dc magnetic field required.
• Compact size.
• Can be used as an oscillator only.
• Low output power and low efficiency.
• Output frequency can be tuned by Vr , or by
  changing the dimensions of the cavity.

94
Travelling-Wave Tube
RF In
RF Out
Helix
Collector
Attenuator
Electron Beam
The TWT is a linear beam device with the
magnetic field running parallel to tube
lengthwise. The helix is also known as a slow
wave structure to slow down the RF field so that
its velocity down the the tube is close to the
velocity of the electron beam.
95
TWT Operation

• As the RF wave travels along the helix, its
  positive and negative oscillations velocity
  modulate the electron beam causing the electrons
  to bunch up.
• The prolonged interaction between the RF wave and
  electron beam along the TWT results in
  exponential growth of the RF voltage.
• The amplified wave is then extracted at the
  output.
• The attenuator prevents reflected waves that can
  cause oscillations.

96
Notes On TWTs

• Since interaction between the RF field and the
  electron beam is over the entire length of the
  tube, the power gain achievable is very high (gt
  50 dB).
• As TWTs are nonresonant devices, they have wider
  bandwidths and lower NF than klystrons.
• TWTs operate from 0.3 to 50 GHz.
• The Twystron tube is a combination of the TWT and
  klystron. It gives better gain and BW over
  either the conventional TWT or klystron.

97
Radio- Wave In Free Space
Radio waves propagate as TEM waves in free space.
For an isotropic (i.e. omnidirectional) source
where PD power density (W/m2) E
electric field intensity (V/m) Pr total
radiated power (W) and d distance from source
(m).
98
Optical Properties Of Radio Waves

• Since light waves and radio waves are part of the
  electromagnetic spectrum, they behave similarly.
• Thus, radio waves can
• refract at the boundary between two different
  media
• reflect at the surface of a conductor
• diffract around the edge of an obstacle
• interfere with one and another to degrade
  performance
• Propagation of radio wave in the atmosphere is
  greatly influenced by the frequency of the wave.

99
Radio Wave Propagation Modes

• In every terrestrial radio system, there are
  three possible modes of propagation
• Ground-wave or surface-wave propagation
• Space-wave or direct-wave propagation
• Sky-wave propagation
• At frequencies lt 2 MHz, ground wave is best.
• Sky waves are used for HF signals.
• Space waves are used for VHF and above.

100
Ground-Wave Propagation
Ground waves start out with the electric field
being perpendicular to the ground. Due to the
gradient density of the earths atmosphere the
wavefront tilts progressively.
Direction of wave travel
Wavefront
Increasing Tilt
Earth
101
Notes On Ground Waves

• Advantages
• Given enough power, can circumnavigate the earth.
• Relatively unaffected by atmospheric conditions.
• Disadvantages
• Require relatively high transmission power.
• Require large antennas since frequency is low.
• Ground losses vary considerably with terrain.
• Applications MF broadcasting ship-to-ship and
  ship-to-shore comms radio navigation maritime
  comms.

102
Space-Wave Propagation
Most terrestrial communications in the VHF or
higher frequency range use direct, line-of-sight,
or tropospheric radio waves. The approximate
maximum distance of communication is given by
where d max. distance in km hT height of the
TX antenna in m hR height of the RX antenna in m
103
Notes On Space-Waves

• The radio horizon is greater than the optical
  horizon by about one third due to refraction of
  the atmosphere.
• Reflections from a relatively smooth surface,
  such as a body of water, could result in partial
  cancellation of the direct signal - a phenomenon
  known as fading. Also, large objects, such as
  buildings and hills, could cause multipath
  distortion from many reflections.

104
Sky-Wave Propagation

• HF radio waves are returned from the F-layer of
  the ionosphere by a form of refraction.
• The highest frequency that is returned to earth
  in the vertical direction is called the critical
  frequency, fc.
• The highest frequency that returns to earth over
  a given path is called the maximum usable
  frequency (MUF). Because of the general
  instability of the ionosphere, the optimum
  working frequency (OWF) 0.85 MUF, is used
  instead.

105
Formulas For Sky Waves

• From geometry (assuming flat earth)
• d 2hv tan qi
• where hv virtual height of F-layer
• From theory (secant law)
• MUF fc sec qi

F-Layer
qi
hv
Earth
d
106
Free-Space Path Loss

• Defined as the loss incurred by a radio wave as
  it travels in a straight line through a vacuum
  with no absorption or reflection of energy from
  nearby objects.
• Formula Lp (dB) 92.4 20log f 20log d where
  f frequency of radio wave in GHz and d
  distance in km.
• If f is in MHz, replace 92.4 above by 32.4.

107
Fade Margin

• To account for changes in atmospheric conditions,
  multipath loss, and terrain sensitivity, a fade
  margin, Fm, must be added to total system loss
• Fm (dB) 30log d 10log(6ABf) - 10log(1-R) -70
• where d distance (km), f frequency (GHz), R
  reliability (decimal value), A terrain
  roughness factor (0.25 to 4), and B factor to
  convert worst-month probability to annual
  probability (0.125 to 1 depending on humidity or
  dryness).

108
Antenna Basics

• An antenna is a passive reciprocal device.
• It acts as a transducer to convert electrical
  oscillations in a transmission line or waveguide
  to a propagating wave in free space and vice
  versa.
• It functions as an impedance matcher between a
  transmission line or waveguide and free space.
• All antennas have a radiation pattern which is a
  plot of the field strength or power density at
  various angular positions relative to the antenna.

109
Antenna Efficiency
An antenna has an equivalent radiation
resistance, Rr given by
where Pr power radiated and i antenna current
at feedpoint
All the power supplied to the antenna is not
radiated.
Antenna efficiency
where Pd power dissipated and Re effective
antenna resistance.
110
Directive Gain Power Gain
Directive gain of an antenna is given by
where PD power density at some point with a
given antenna PDr power density at the same
point with a reference antenna.
Maximum directive gain is called directivity.
Reference antenna is generally the isotropic
source.
When antenna efficiency is taken into
account directive gain becomes power gain Ap ?
D. In decibels, power gain is 10 log Ap
111
Effective Isotropic Radiated Power
EIRP is the equivalent power that an isotropic
antenna would have to radiate to achieve the
same power density at a given point as another
antenna EIRP PrAt PinAp where Pr total
radiated power Pin antenna input power At
TX antenna directive gain and Ap antenna power
gain.
Therefore, the power density at a distance, d,
from an antenna is
112
Antenna Miscellany

• Power captured by the receiving antenna with an
  effective area, Aeff, is C PDAeff. Note that
  Aeff includes the gain and efficiency of the
  antenna.
• Antennas can be linearly, elliptically or
  circularly polarized depending on their E-field
  radiated.
• Antenna beamwidth is the angular separation
  between the two half-power points on the major
  lobe of the antennas plane radiation pattern.
• Antenna input impedance, Zin Ei/Ii

113
Half-Wave Dipole
?/2
Symbol
Balanced Feedline

• Simple and most widely used at f gt 2 MHz.
• Its a resonant antenna since its length is 2 x
  l/4.
• Zin 73 W approx. Zmax 2500 W approx. at ends
• Radiation pattern of dipole in free space has two
• main lobes perpendicular to the antenna axis.
• Has a gain of about 2.15 dBi

114
Free-Space Radiation Pattern of Dipole
115
Ground Length Effects On Dipole

• Since the ground reflects radio waves, it has a
  significant effect on the radiation pattern and
  impedance of the half-wave dipole.
• Generally speaking, the closer the dipole is to
  the ground, the more lobes will form and the
  lower the radiation impedance.
• Length also has an effect on the dipole antenna
  dipoles shorter than l/2 is capacitive while
  dipoles longer than l/2 is inductive.

116
Marconi/Monopole Antenna

• Main characteristics
• vertical and l/4
• good ground plane is required
• omnidirectional in the horizontal plane
• 3 dBd power gain
• impedance about 36W

117
Antenna Impedance Matching

• Antennas should be matched to their feedline for
  maximum power transfer efficiency by using an LC
  matching network.
• A simple but effective technique for matching a
  short vertical antenna to a feedline is to
  increase its electrical length by adding an
  inductance at its base. This inductance, called
  a loading coil, cancels the capacitive effect of
  the antenna.
• Another method is to use capacitive loading.

118
Antenna Loading
Capacitive Loading
Inductive Loading
119
Antenna Arrays

• Antenna elements can be combined in an array to
  increase gain and get desired radiation pattern.
• Arrays can be classified as broadside or
  end-fire, according to their direction of maximum
  radiation.
• In a phased array, all elements are fed or
  driven i.e. they are connected to the feedline.
• Some arrays have only one driven element with
  several parasitic elements which act to absorb
  and reradiate power radiated from the driven
  element.

120
Yagi-Uda Array

• More commonly known as the Yagi array, it has one
  driven element, one reflector, and one or more
  directors.

Radiation pattern
121
Characteristics of Yagi Array

• unidirectional radiation pattern (one main lobe,
  some sidelobes and backlobes)
• relatively narrow bandwidth since it is resonant
• 3-element array has a gain of about 7 dBi
• more directors will increase gain and reduce the
  beamwidth and feedpoint impedance
• a folded dipole is generally used for the driven
  element to widen the bandwidth and increase the
  feedpoint impedance.

122
Folded Dipole

• Often used - alone or with other elements - for
  TV and FM broadcast receiving antennas because it
  has a wider bandwidth and four times the
  feedpoint resistance of a single dipole.

l 2
Zin 288 W
Feed line
123
Log-Periodic Dipole Array (LPDA)
D6
D5
L5
L4
L3
L2
a
L6
Apex
Feed line
Direction of main lobe
124
Characteristics of LPDA

• feedpoint impedance is a periodic function of log
  f
• unidirectional radiation and wide bandwidth
• shortest element is less than or equal to l/2 of
  highest frequency, while longest element is at
  least l/2 of lowest frequency
• reasonable gain, but lower than that of Yagi for
  the same number of elements
• design parameter, t L1/L2 D1/D2 L2/L3 .
• used mainly as HF, VHF, and TV antennas

125
Turnstile Array
Half-wave dipoles fed 90o out-of phase

• omnidirectional radiation in the horizontal
  plane, with horizontal polarization
• gain of about 3 dB less than that of a single
  dipole
• often used for FM broadcast RX and TX

126
Collinear Array

• all elements lie along a straight line, fed in
  phase, and often mounted with main axis vertical
• result in narrow radiation beam omnidirectional
  in the horizontal plane when antenna is vertical

Half-wave Elements
Feed Line
Quarter-wave Shorted Stub
127
Broadside Array

• all l/2 elements are fed in phase and spaced l/2
• with axis placed vertically, radiation would have
  a narrow bidirectional horizontal pattern

Half-wave Dipoles
Feed Line
l 2
128
End-Fire Array

• dipole elements are fed 90o out of phase
  resulting in a narrow unidirectional radiation
  pattern off the end of the antenna

Feed Line
l 4
Half-wave Dipoles
RadiationPattern
129
Non-resonant Antennas

• Monopole and dipole antennas are classified as
  resonant type since they operate efficiently only
  at frequencies that make their elements close to
  l/2.
• Non-resonant antennas do not use dipoles and are
  usually terminated with a matching load resistor.
• They have a broader bandwidth and a radiation
  pattern that has only one or two main lobes.
• Examples of non-resonant antennas are long-wire
  antennas, vee antennas, and rhombic antennas.

130
Loop Antenna

• Main characteristics
• very small dimensions
• bidirectional
• greatest sensitivity in the plane of the loop
• very wide bandwidth
• efficient as RX antenna with single or
  multi-turn loop

Feedline
131
Helical Antenna
D

• broadband ( 20 of fo)
• circularly polarized
• Ap 15 dB q-3dB 20o are typical
• when S, D, of turns increase
• Ap increases and q decreases
• to get higher gain and narrower
• beamwidth, use an array
• applications V/UHF antenna
• satellite tracking antenna

S
Ground Plane
Coaxial Feedline
End-fire Helical Antenna
132
UHF Microwave Antennas

• highly directive and beamwidth of about 1o or
  less
• antenna dimensions gtgt wavelength of signal
• front-to-back ratio of 20 dB or more
• utilize parabolic reflector as secondary antenna
  for high gain
• primary feed is either a dipole or horn antenna
• use for point-to-point and satellite
  communications

133
Parabolic Reflector Antenna
Power gain and -3 dB beamwidth are
where h antenna efficiency (0.55 is
typical) D dish diameter (m) and l
wavelength (m)
134
Hog-horn Antenna

• The hog-horn antenna, often used for terrestrial
  microwave links, integrates the feed horn and a
  parabolic reflecting surface to provide an
  obstruction-free path for incoming and outgoing
  signals.

Parabolic Section
Feed Horn
135
Microwave Radio Communications

• Can be classified as either terrestrial or
  satellite systems.
• Early systems use FDM (frequency division
  multiplex) technique.
• More recent systems use PCM/PSK (pulse code
  modulation/phase shift keying) technique.
• Microwave system capacities range from less than
  12 VB (voice-band) channels to gt 22,000.
• Operate from 24 km to 6,400 km.

136
Simplified Block Diagram
Upconverter
Preemphasized Baseband Input
FM Modulator
Mixer
BPF
RF Out
Amp
Ch. Combiner
RF Oscillator
IF Oscillator
FM Microwave Transmitter
Downconverter
Deemphasized Baseband Output
FM Detector
Mixer
BPF
RF In
Amp
Ch. Separator
RF Oscillator
FM Microwave Receiver
137
Notes On FM Microwave Radio System

• Baseband signals may comprise one or more of
• Frequency-division-multiplexed voice-band
  channels
• Time-division-multiplexed VB channels
• Broadcast-quality composite video or picturephone
• Wideband data
• IF carrier is typically 70 MHz
• Low-index frequency modulation is used
• Common microwave frequencies used 2-, 4-, 6-,
  12-, and 14-GHz bands.

138
Microwave Radio Systems (contd)

• The distance between transmitter and receiver is
  typically between 24 to 64 km.
• Repeaters have to be used for longer distances.
• To increase the reliability of microwave links,
  the following techniques can be used
• frequency diversity - two RF carrier frequencies
• space diversity - two or more antennas are used
• polarization diversity - vertical and horizontal
  polarization

139
System Gain

• System gain for microwave radio link is
• Gs (dB) Pt - Cmin Fm Lp Lf Lb - At - Ar
• where Pt transmitter output power (dBm)
• Cmin min. receiver input power (dBm)
• Fmfade margin for a given reliability objective
  (dB)
• Lp free-space path loss between antennas (dB)
• Lf, Lb feeder, coupling, branching losses
  (dB)
• At, Ar Tx and Rx antenna gain respectively (dB)

140
Introduction To Pulsed Radar
Pulse of energy
t
Pulse Repetition Time
PRT
Pulse repetition frequency, PRF 1/PRT
Duty cycle, D t/PRT
Range to target, R ct/2, where c speed of
light, and t time between TX pulse and echo
return.
Dead zone, Rdead, and resolution, DR, are both
ct/2.
Resolution can be improved by pulse compression.
141
Radar Power Range Equation
Average power, Pa Ppt(PRF) Ppt/PRT
PpD where Pp peak power.
Ideal radar range equation
where PR signal power returned (W) G antenna
gain l wavelength of signal (m) s radar cross
section of target (m2)
In the real world, losses and noise must be
added to above equation.
142
Pulsed Radar Block Diagram
Receiver Section
Video Amp
Video Detector
IF Amp
Mixer
RF Amp
Antenna
LO
Signal Processor
T/R Switch
Control Section
Modulator Timer
Transmitter
Display
143
Radar Display Modes
N
Beam Sweep
Targets
Target
Range
Elevation
Plan Position Indicator
E-Scan
144
CW Doppler Radar
The Doppler effect can be used for determining
the speed of a moving target.
Microwave Oscillator
TX RX
Circulator
Doppler Mixer
v lfd/2 (m/s) where fd doppler shift (Hz) l
radar wavelength (m)
fd
Basic block diagram of CW Doppler radar
145
FM Doppler Radar
Both distance and velocity can be determined
if an FM Doppler radar is used.
fi
Range
TX
where a slope of line or rate of change of fi
fd-
fo
RX
fd
Velocity
t
146
Optical Fibre Communications

• Advantages over metallic/coaxial cable
• much wider bandwidth and practically
  interference-free
• lower loss and light weight
• more resistive to environmental effects
• safer and easier to install
• almost impossible to tap into a fibre cable
• potentially lower in cost over the long term
• Disadvantages
• higher initial cost in installation more
  expensive to repair/maintain

147
Optical Fibre Link
Transmitter
Input Signal
Coder or Converter
Light Source
Source-to-fibre Interface
Fibre-optic Cable
Output
Light Detector
Fibre-to-light Interface
Amplifier/Shaper Decoder
Receiver
148
Types Of Optical Fibre
Light ray
n1 core
n2 cladding
Single-mode step-index fibre
no air
n1 core
n2 cladding
Multimode step-index fibre
no air
Variable n
Multimode graded-index fibre
Index porfile
149
Comparison Of Optical Fibres

• Single-mode step-index fibre
• minimum signal dispersion higher TX rate
  possible
• difficult to couple light into fibre highly
  directive light source (e.g. laser) required
  expensive to manufacture
• Multimode step-index fibres
• inexpensive easy to couple light into fibre
• result in higher signal distortion lower TX rate
• Multimode graded-index fibre
• intermediate between the other two types of fibres

150
Acceptance Cone Numerical Aperture
n2 cladding
Acceptance Cone
qC
n1 core
n2 cladding
Acceptance angle, qc, is the maximum angle in
which external light rays may strike the
air/fibre interface and still propagate down the
fibre with lt10 dB loss.
Numerical aperture NA sin qc ?(n12 - n22)
151
Losses In Optical Fibre Cables

• The predominant losses in optic fibres are
• absorption losses due to impurities in the fibre
  material
• material or Rayleigh scattering losses due to
  microscopic irregularities in the fibre
• chromatic or wavelength dispersion because of the
  use of a non-monochromatic source
• radiation losses caused by bends and kinks in the
  fibre
• modal dispersion or pulse spreading due to rays
  taking different paths down the fibre
• coupling losses caused by misalignment
  imperfect surface finishes

152
Absorption Losses In Optic Fibre
6
Rayleigh scattering ultraviolet absorption
5
4
Loss (dB/km)
3
Peaks caused by OH- ions
Infrared absorption
2
1
0
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Wavelength (mm)
153
Fibre Alignment Impairments
Axial displacement
Gap displacement
Angular displacement
Imperfect surface finish
154
Light Sources

• Light-Emitting Diodes (LED)
• made from material such as AlGaAs or GaAsP
• light is emitted when electrons and holes
  recombine
• either surface emitting or edge emitting
• Injection Laser Diodes (ILD)
• similar in construction as LED except ends are
  highly polished to reflect photons back forth

155
ILD versus LED

• Advantages
• more focussed radiation pattern smaller fibre
• much higher radiant power longer span
• faster ON, OFF time higher bit rates possible
• monochromatic light reduces dispersion
• Disadvantages
• much more expensive
• higher temperature shorter lifespan

156
Optical Transmitter Circuits
VCC
C1
R2
Data Input
Q1
HV
R1
LED
Enable
C1
R3
Q1
Data Input
C2
R1
Enable
R2
ILD
157
Light Detectors

• PIN Diodes
• photons are absorbed in the intrinsic layer
• sufficient energy is added to generate carriers
  in the depletion layer for current to flow
  through the device
• Avalanche Photodiodes (APD)
• photogenerated electrons are accelerated by
  relatively large reverse voltage and collide with
  other atoms to produce more free electrons
• avalanche multiplication effect makes APD more
  sensitive but also more noisy than PIN diodes

158
Photodetector Circuit
V
R1
Comparator shaper
Data Out
-
-
PIN or APD


Enable
-

Threshold adjust
159
Bandwidth Power Budget

• The maximum data rate R (Mbps) for a cable of
  given distance D (km) with a dispersion d (ms/km)
  is
• R 1/(5dD)
• Power or loss margin, Lm (dB) is
• Lm Pr - Ps Pt - M - Lsf - (DxLf) - Lc - Lfd
  - Ps ? 0
• where Pr received power (dBm), Ps receiver
  sensitivity(dBm), Pt Tx power (dBm), M
  contingency loss allowance (dB), Lsf
  source-to-fibre loss (dB), Lf fibre loss
  (dB/km), Lc total connector/splice losses (dB),
  Lfd fibre-to-detector loss (dB).