radio & telecommunications systems 1 radio & telecommunications systems (1.0) lecturer: p.m....

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)


• 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


• 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


• 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%


• 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


• 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 Wave Propagation

• Radio wave characteristics

• Radiation from an antenna

• Propagation characteristics


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


• 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


Figure 1 - Sine wave characteristic

+

-

1 cycleperiod

A B C Dtime

Positive

alternation

Negative

alternation1 second


Figure 2 - Concept of a wavelength

wavelength

distance


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)


Figure 3 - Electromagnetic lines of force

Electric

lines

of force

Magnetic

lines

of force


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


Figure 4A - Vertically polarized waveVertical

antenna

Wavefront

Earth

Electric lines

Magnetic lines


Figure 4B - Horizontally polarized waveHorizontal

antenna

Wavefront

Earth

Electric lines

Magnetic lines


Radio & Telecommunications Systems

Induction/Radiation Field

Free Space Impedance

Modes of Propagation


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


Figure 1 - Electromagnetic wave

Distance

Magne

tic fie

ld

Ele

ctric

fiel

d

90 o

90 o

90 o


Figure 5 - Radiation from an aerial

Closed loops of

magnetic flux

Closed loops of

electric flux

Aerial


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


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


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


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


Figure 2 - Ground wave propagation

Transmitting

antenna

Earth

Ground

waves


• 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


Figure 3 - Space wave propagation

Transmitting

antenna

Earth

Space

waves

Receiving

antenna


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 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


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.


Figure 4 - Sky wave propagation

Transmitting

antenna

Earth

E

F1

F2

D

Ionosphere

Iono

nsph

eric

laye

rs

Reflectedsignal


• 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


• 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.


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)


• 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


• 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


• 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


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.


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


• 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


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


• 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


Figure 3 - Effect on ionospheric refraction

Earth

12

30MHz

30MHz

20MHz

20MHz

10MHz5MHz

5MHz

10MHz

E layer

F 1 layer

F 2 layer


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


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...


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


Reference

• Dungan F.R., “Electronic Communications Systems,” 3rd ed., ITP

• Green D.C., “Radio Systems for Technicians,” Longman

radio & telecommunications systems 1 radio & telecommunications systems (1.0) lecturer: p.m....

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