Download - Radio & Telecommunications Systems (1.0)
Radio & Telecommunications Systems 1
Radio & Telecommunications Systems (1.0)
Lecturer: P.M. Cheung (room 326)
email:[email protected]
Contact Hours: Lecture: 30 hours (room 310)
Tutorial: 15 hours (room324)
Lab. :4 experiments(room 324)
Radio & Telecommunications Systems 2
• Content: 1. EM wave & Antenna • 2. Transmitters & Receivers• 3. Telephone systems• 4. TV Systems
• Assessment: Course work – 50%• (assignment:10%, lab:20%, test: 20%)
• Exam. - 50%• Textbook: Electronic Communications Systems, 3rd
ed.,Dungan, Delmar• No lecture notes will be delivered.
• Download from intranet: http://172.26.126.61/student
Radio & Telecommunications Systems 3
• Aims
• establish an understanding of the elementary principles employed in radio transmitter and receiver systems
• introduce the basic system knowledge of various kinds of local telecommunications systems
• Co-requisites
• Telecommunications Principles 1
Radio & Telecommunications Systems 4
• Learning Strategies
• emphasis on the general aspects and appreciation of radio and telecommunications systems
• practical examples of various telecommunications systems will be used to promote learning
• Assessment
• Continuous assessment - 50%
• Examination - 50%
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• Content Area
• Electromagnetic wave and antenna systems
• radiation of electromagnetic wave
• modes of propagation
• parameters of aerial
• practical aerials
• Radio transmitters and receivers
• block diagrams of : AM transmitters, FM transmitters, superheterodyne receiver
• diode detector
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• Telephone systems
• fixed network technology, space and time switching
• local loop, signalling in call establishment
• overview of mobile communications; cellular communications, multiple access
• Television systems
• scanning, composite video, PAL/NTSC systems, TV transmission/reception
• overview of satellite boardcast; orbit, earth station, satellite TV
Radio & Telecommunications Systems 7
Radio Wave Propagation
• Radio wave characteristics
• Radiation from an antenna
• Propagation characteristics
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Radio Wave Characteristics
• the radiation concept of radio waves
• dropping a pebble into a pool of water
• water to move up and down
• disturbance transmitted as expanding circles of waves
• transverse wave or traveling wave
• occurring perpendicular to the direction of propagation.
• e.g. electromagnetic waves radiated by antennas
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• Frequency• the number of cycles of a sine wave completed in
one second expressed in Hz (Figure 1)• Radio Frequencies (RF)
• frequencies between 3 kHz and 300 GHz • commonly used in radio communication.
• Wavelength (• the space occupied by one full cycle of a radio
wave at any given instant (Figure 2)
= c / f
c = velocity of radio wave = 3x108m/s
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Figure 1 - Sine wave characteristic
+
-
1 cycleperiod
A B C Dtime
Positive
alternation
Negative
alternation1 second
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Figure 2 - Concept of a wavelength
wavelength
distance
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Electromagnetic Radiation
• complex form of energy containing both electric and magnetic fields
• moving electric field always creates a magnetic field
• moving magnetic field always creates an electric field
• lines of force of these fields are perpendicular to each other (Figure 3)
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Figure 3 - Electromagnetic lines of force
Electric
lines
of force
Magnetic
lines
of force
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Wave Polarization
• determined by the direction of the electric field of the wave with respect to earth
• vertically polarized
• electric field of the wave is vertical to the earth (Figure 4A)
• horizontally polarized
• electric field is horizontal to the earth (Figure 4B)
• position of the transmitting antenna determines whether the wave will be vertically or horizontally polarized
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Figure 4A - Vertically polarized waveVertical
antenna
Wavefront
Earth
Electric lines
Magnetic lines
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Figure 4B - Horizontally polarized waveHorizontal
antenna
Wavefront
Earth
Electric lines
Magnetic lines
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Radio & Telecommunications Systems
Induction/Radiation Field
Free Space Impedance
Modes of Propagation
Radio & Telecommunications Systems 18
Radiated field
• energy radiated from the conductor or aerial
• in the form of an electromagnetic wave
• electric and magnetic fields are at right angles to each other
• mutually at right angles to the direction of propagation (Figure 1)
• magnitude proportional to the frequency of the wave and inversely proportional to the distance from the aerial
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Figure 1 - Electromagnetic wave
Distance
Magne
tic fie
ld
Ele
ctric
fiel
d
90 o
90 o
90 o
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Figure 5 - Radiation from an aerial
Closed loops of
magnetic flux
Closed loops of
electric flux
Aerial
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Induction field
• near the aerial
• energy that is not radiated away from the aerial
• magnitude diminishes inversely as the square of the distance from the aerial
• the induction field larger than the radiation field
• at distances greater than /2• radiation field is the larger
Radio & Telecommunications Systems 22
Free Space Impedance
• amplitudes of electric field E & magnetic field H constant relationship to each other.
Impedance of free space
=E (volts/meter) / H (ampere-turns/meter)
=120
=377
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Propagation Characteristics• electromagnetic wave sent out from an antenna
• ground wave• part of the radiated energy travels along or near the surface of the earth
• sky wave• another part travels from the antenna upward into space
• space wave• energy that travels directly from the transmitting antenna to the receiving antenna
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Ground Waves
• primary mode of propagation in
• LF band (30 - 300 KHz)
• MF band (300 KHz - 3MHz)
• follow the curvature of the earth and actually travel beyond the horizon (Figure 2)
• as the frequency increases
• more effectively absorbed by the irregularities on the earth's surface
• hills, mountains, trees, and buildings
Radio & Telecommunications Systems 25
Figure 2 - Ground wave propagation
Transmitting
antenna
Earth
Ground
waves
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• Space Waves• transmitted signal above 4 or 5 MHz
• usable ground wave signal is limited to a few miles.
• signals can be transmitted farther using the space or direct wave (Figure 3)
• used primarily in • VHF band (30 - 300 MHz)• UHF band (300 MHz - 3 GHz)
• limited to line-of-sight distances• energy in radio waves at frequencies above 30
MHz moves through space in straight lines like light waves
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Figure 3 - Space wave propagation
Transmitting
antenna
Earth
Space
waves
Receiving
antenna
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Radio Horizon
• about one third greater than that of the optical horizon
• caused by refraction in the earth's lower atmosphere
• density of the earth's atmosphere decreases linearly as height increases
• effectively bending the wave slightly downward
• follows the curvature of the earth beyond the optical horizon
Radio & Telecommunications Systems 29
• radio horizon for both transmitting and receiving antennas:
Dt = 4t or Dr = 4r
where Dt and Dr = radio horizon distance in kilometers
Ht and Hr = height of transmitting (receiving) antenna in meters
• maximum space wave communications distance is the sum of the numbers obtained by for both antennas.
Dmax = 4t + 4r or Dmax =Dt + Dr
Radio & Telecommunications Systems 30
Sky Waves
• ionized layers of the atmosphere between 50 - 400km above the surface of the earth
• at certain frequencies and radiation angles• the ionosphere reflects radio waves• radio waves at other frequencies and angles
are refracted and return to earth (Figure 4)• amount of refraction depends on
• frequency of the wave• density of the ionized layer• angle at which the wave enters the
ionosphere.
Radio & Telecommunications Systems 31
Figure 4 - Sky wave propagation
Transmitting
antenna
Earth
E
F1
F2
D
Ionosphere
Iono
nsph
eric
laye
rs
Reflectedsignal
Radio & Telecommunications Systems 32
• long distance communications
• carrier frequencies in the MF and HF bands (3 - 30 MHz)
• waves radiated at these frequencies can be refracted back to earth
• waves at frequencies above 30 MHz
• penetrate the ionosphere and continue moving out into space
Radio & Telecommunications Systems 33
• The Ionosphere:
• atmospheric conditions continuously change
• hourly, daily, monthly, seasonally, yearly…..
• undesirable results are
• signal absorption, dispersion and fading
• atmospheric conditions have their greatest effect on the ionosphere
• graphic illustration of the designations of the ionospheric layers and their approximate altitudes is shown in Figure 1.
Radio & Telecommunications Systems 34
Figure 1 - Layers in the ionosphere
F 2
F 1
E
D
400km
250km
220/200km150km
90km
50km
Earth
Electronic density(electrons/m 3)
Hei
ght a
bove
gro
und
F 2 layer
F 1 layer
E layer
D layer
(a) (b)
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• D layer, 50-90 km above the earth
• lowest layer
• exists only in the daytime
• ionization is relatively weak
• does not affect the travel direction of radio waves
• absorb energy from the electromagnetic wave
• attenuates the sky wave
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• MF band signals are completely absorbed by the D layer
• at night
• D layer disappears
• long distance MF transmissions via sky wave
• E layer, 90-150 km above earth
• maximum density at noon
• ionization is so weak at night
• the layer may disappear
Radio & Telecommunications Systems 37
• F layer, 200-400 km above earth
• splits into F1 and F2 in the daytime
• F2 varying from summer to winter
• F1 layer, 200-220 km above the surface of the
earth.
• F2 layer, 250-350km in winter, 300-500km in
summer
Radio & Telecommunications Systems 38
Frequency bands and major services
Band Range Major ServicesVLFLF
10-30kHz30-300KHz
Radio navigation; time and frequency broadcasts; maritimemobile communications; aeronautical communications
MF 300kHz-3MHz AM broadcasting; amateur communications; time andfrequency broadcasts; fixed and mobile communications;maritime and aeronautical aids and communications
HF 3-30MHz Shortwave broadcasting; time and frequency broadcasts;point-to-point communication; amateur communications;land, maritime, and aeronautical communications
VHF 30-300MHz Land and aeronautical mobile communications; industrialand amateur communications; FM and TV broadcasting;space and meteorological communications; radio navigation
UHF 300MHz-3GHz*
TV broadcasting; aeronautical and land mobilecommunications; radioastronomy; telemetry; satellitecommunications; amateur communications
SHFEHF
3GH-30GHz*30GHz-300GHz*
Microwave relay; satellite and exploratory communications;amateur communications
* Frequencies above about 900 MHz are considered to be microwaves.
Radio & Telecommunications Systems 39
Refraction of an Electromagnetic Wave
• electromagnetic wave travelling in one medium passes into a different medium
•direction of travel will probably be altered
•wave is said to be refracted.
• The ratio
is a constant for a given pair of media and is known as the refractive index for the media.
r
in
sinsin
Radio & Telecommunications Systems 40
• wave is transmitted through a number of thin strips (Figure 2)
• each strip having an absolute refractive index lower than that of the strip immediately below it
• wave will pass from higher to lower absolute refractive
• progressively bent away from the normal• wave will be continuously refracted
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Figure 2 - Refraction of an electromagnetic wave
7
6
4
5
3
2
1
Path of wave
through strips
Strip
s of
diff
erin
g ab
solu
tere
fract
ive
inde
x
Radio & Telecommunications Systems 42
• refractive index n of a layer is related to both the frequency f of the wave and the electron density N according to:
281
1sinsin
f
Nn
r
i
Radio & Telecommunications Systems 43
Figure 3 - Effect on ionospheric refraction
Earth
12
30MHz
30MHz
20MHz
20MHz
10MHz5MHz
5MHz
10MHz
E layer
F 1 layer
F 2 layer
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Critical Frequency
• the max frequency that can be radiated vertically upwards by a radio transmitter and be returned to earth
• wave that travels to the top of the layer• electron density is at its maximum value• angle of refraction becomes 90o
• angle of incidence is 0o
• therefore
max
2max
9
81100sin
Nf
f
N
crit
crit
o
Radio & Telecommunications Systems 45
Maximum Usable Frequency (MUF)• highest frequency that can be used to establish
communication• using the sky wave• between two points
• determined by both the angle of incidence of the radio wave and the critical frequency of the layer; thus
i
critffum
cos...
Radio & Telecommunications Systems 46
Optimum Working Frequency (OWF)
• ionospheric fluctuations often take place• operation of a link at the m.u.f. would not be
reliable• frequency of about 85% of the m.u.f.
• used to operate a sky-wave link• known as the optimum working frequency or
o.w.f• since the m.u.f. will vary over the working day
• necessary to change the transmitter frequency as propagation condition varies
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Reference
• Dungan F.R., “Electronic Communications Systems,” 3rd ed., ITP
• Green D.C., “Radio Systems for Technicians,” Longman