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Senior Science
HSC Course
Stage 6
Information systems
Part 2: Waves waves waves
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Number: 43170
Title: Information systems
This publication is copyright New South Wales Department of Education and Training (DET), however it may contain
material from other sources which is not owned by DET. We would like to acknowledge the following people and
organisations whose material has been used:
Text of interview with scientist Sue Spaargaren, (accessed November, 2000) found at
http://www.swimwithdragons.com.au/cgibin/cgi.../allegro.pl?wis_search.Sue+Spaargaren
Part 6 pp 19-22
COMMONWEALTH OF AUSTRALIA
Copyright Regulations 1969
WARNING
This material has been reproduced and communicated to you on behalf of the
New South Wales Department of Education and Training
(Centre for Learning Innovation)
pursuant to Part VB of the Copyright Act 1968 (the Act).
The material in this communication may be subject to copyright under the Act.
Any further reproduction or communication of this material by you may be the
subject of copyright protection under the Act.
All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in good faith.
Published by
Centre for Learning Innovation (CLI)
51 Wentworth Rd
Strathfield NSW 2135
_______________________________________________________________________________________________
_
Copyright of this material is reserved to the Crown in the right of the State of New South Wales. Reproduction or
transmittal in whole, or in part, other than in accordance with provisions of the Copyright Act, is prohibited without the
written authority of the Centre for Learning Innovation (CLI).
© State of New South Wales, Department of Education and Training 2007.
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Contents
Introduction ............................................................................... 2
What is a wave? ........................................................................ 3
What is the source of a wave? ............................................................4
Waves carry energy .............................................................................5
Sound waves............................................................................. 8
The electromagnetic spectrum ................................................ 10
EMS and communication ...................................................................13
Microwave and radio wave use .........................................................17
Summary................................................................................. 20
Appendix ................................................................................ 23
Suggested answers................................................................. 25
Exercises–Part 2 ..................................................................... 27
Part 2: Waves waves waves
1
Introduction
This part shows how electromagnetic waves can be modulated
(adjusted) to carry information. You will become familiar with parts of
the electromagnetic spectrum and the frequencies that each
communication system uses.
In Part 2 you will be given opportunities to learn to:
•
identify the type of waves in the electromagnetic spectrum
currently used for communication systems as
–
visible light
–
infra–red
–
microwaves
–
radio waves, which include:
–
TV
–
FM radio
–
AM radio
•
compare the advantages and disadvantages of using microwaves
and radio waves in communication technologies
•
identify communication technologies that use energies from the
electromagnetic spectrum for communication purposes
•
describe the individual properties of visible light, radio waves
(AM, FM, TV waves) and microwaves and relate these to their use
in communication systems.
Extracts from Senior Science Stage 6 Syllabus © Board of Studies NSW,
November 2002. The most up–to–date version is to be found at
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/index.html
2
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What is a wave?
In a communication device, a signal must be carried by something.
If no wires connect two communication devices, the signal must be
carried by a wave.
You are probably familiar with the waves you see at the beach.
The waves in the ocean, that is the ones that aren’t crashing on the
beach, are the shape of the types of waves you will be investigating.
If you were able to look at these waves side–on, you would see they are
in the shape of the wave diagram below. This is just a model of a
wave–not all waves look like the one below.
Standard wave.
•
The highest point of a wave is the crest.
•
The lowest point of a wave is a trough.
•
The distance from crest to crest or trough to trough is one
wavelength.
•
The number of waves to pass a point (like a lighthouse) in one
second is called the wave frequency.
1
On the above diagram, label the following:
a) crest
b) trough
c) wavelength
Part 2: Waves waves waves
3
2
Frequency is the number of wavelengths to pass a point in one
second. The units of frequency are hertz (Hz).
a) If 10 waves pass a point in one second, what is the in
frequency?
__________________________________________________
b) If 25 waves pass a point in one second, what is the in
frequency?
__________________________________________________
c) If 300 waves pass a point in one second, what is their
frequency?
__________________________________________________
Check your answers.
You are not required to calculate wave frequencies in this course,
however this activity will help you understand wave classification
based on wave frequencies later in this part.
What is the source of a wave?
So what actually produces a wave? Do you have any ideas?
You probably know that when you throw something into a still body of
water, like a lake or puddle, that waves are produced. This is due to the
initial disturbance of the water particles, or the initial vibration.
Imagine you have set up the apparatus in the following diagram.
A mass is attached to a hanging spring and a pen is attached to the
mass. If you pulled on the mass then let it go, the mass would bob up
and down. The pen will mark out a vertical straight line on the paper as
the mass is in motion.
vibration
direction
straight line
paper is stationary
Mass
on the end of a spring in motion.
4
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Now imagine the paper is moving at a constant speed past the pen as the
mass is bobbing up and down at a constant rate. The pen will mark out
the following wave.
vibration
direction
trace left by pencil is a sine wave shape
paper moving at constant speed
Mass on the end of a spring in motion. A pencil attached to the mass is
marking out a wave as the paper moves past the pencil at a constant rate.
You should notice that:
•
the mass is the vibrating object
•
the mass moves up and down (not forward and back)
•
the wave produced is at right angles to the motion of the mass
•
the vibration causes a wave form.
Waves are caused by a vibrating object or particle.
Waves carry energy
If you have ever been dumped by a wave at the beach, you would have
felt the energy of the wave. Even small waves carry energy from one
place to another.
Different types of waves carry different types of energy and therefore
different types of information. For this reason, waves are classified
according to their properties.
There are two main groups of waves. They are electromagnetic waves
and mechanical waves. The characteristics of each type of wave are
outlined in the chart on the following page.
Part 2: Waves waves waves
5
Waves
electromagnetic
mechanical
do not require medium
for transmission
do require medium
for transmission
all transverse
alternating
electric and
magnetic fields
operating
perpendicularly
to the direction
of wave travel
transverse
particles vibrate
perpendicularly
to the direction
of the wave
propagation
longitudinal
particles vibrate
in the same
direction as
wave
propagation
Different wave types can be classified according to the energy they comprise
or the source of the vibration (or disturbance) producing the wave.
1
Which waves do not require a medium (such as a solid, liquid or gas)
for transmission?
_____________________________________________________
2
Which types of wave transfers energy perpendicular to an electric field
or particle movement?
_____________________________________________________
3
Which wave type is produced by particles vibrating in the same
direction as the flow of energy?
_____________________________________________________
4
Which wave type requires a medium (such as a solid, liquid or gas) for
wave transmission?
_____________________________________________________
Check your answers before moving on.
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A vacuum is a space that contains no particles of matter. This means it
is entirely empty of solids, liquids and gas. Outer space is a natural
example of a vacuum as it contains no matter.
5
Can electromagnetic waves travel through a vacuum? Explain
why or why not.
_____________________________________________________
_____________________________________________________
6
Can mechanical waves travel through a vacuum? Explain why or
why not.
_____________________________________________________
_____________________________________________________
Check your answers.
Mechanical waves can only deliver information over short distances
due to the nature of the waves. Sound waves are an example of
mechanical waves, which will be addressed later.
Many communication systems use electromagnetic waves for
information delivery. These can transfer information quickly and over
enormous distances.
Turn to Exercise 2.1 at the back of this part to practice classifying waves.
Part 2: Waves waves waves
7
Sound waves
Turn to the classification of waves chart on page 6. You are about to
perform an investigation of mechanical longitudinal waves (these on
the right of the chart).
Sound waves are mechanical longitudinal waves. What this means is
that air particles must bump into each other in order for sound to travel.
Sounds in air travel like the compressions in this slinky spring.
compression
rarefaction
rarefaction
compression
The springs compress and rarefact as the wave pulse moves along the spring.
The areas where the spring is compressed is called a compression.
Where the spring has greater space between the springs, it is called a
rarefaction.
In transverse waves, the wavelength is usually measured from crest to
crest. Longitudinal wavelengths are measured from compression to
compression or rarefaction to rarefaction.
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The air particles below show what longitudinal waves look like in air
close up. Notice the compressions and rarefactions in the sound wave.
wavelength
Air particles compress together as sound waves move through air.
Air particles move in the same direction as the sound energy.
Without air particles, sound energy has no medium to be transferred to,
therefore sound energy cannot travel.
Part 2: Waves waves waves
9
The electromagnetic spectrum
Do you know what the electromagnetic spectrum is?
The electromagnetic spectrum (EMS) is responsible for sun burn,
X–rays, everything you see, the heating of food, the music you hear on
the radio, the shows you watch on television and much more.
So what is the electromagnetic spectrum?
The electromagnetic spectrum is a continuum of electromagnetic
waves, which are arranged in order of frequency and wavelength.
If that sounds too technical, the following diagram should help as it
demonstrates the various electromagnetic waves of the electromagnetic
spectrum.
gamma
rays
x-rays ultraviolet
infra-red
light
radio waves
radio
microwaves TV
electrical
power
1 cm
103 km
Wavelength
0.01 nm
1 nm
0.1 mm
0.01 mm
1m
1 km
0.4–0.7mm
The electromagnetic spectrum is arranged in order of increasing wavelength.
Adapted from OTEN, Physics for Electrical and Electronic Engineers.
Did you notice the units ‘nm’, and ‘mm’?
These units indicate nanometres (10–9 m) and micrometres (10–6 m).
It may be useful to think of the different wavelengths in the
electromagnetic spectrum in the following ways.
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Use the diagram on the previous page to answer the following questions.
1
Highlight or underline which of the following has the shorter
wavelength.
a) Gamma rays or television radio waves.
b) Infra–red waves or X–rays.
c) Ultraviolet waves or visible light.
d) Microwaves or infra–red waves.
All electromagnetic waves travel at the same speed which is the speed
of light. If a wave has a small wavelength, more waves are able to pass
a point in one second than a longer wavelength. Keep this in mind as
you answer the following questions.
2
Highlight which of the following are likely to have a higher
frequency? (This means the waves are smaller and therefore more
waves are likely to pass a point in one second.)
a) Microwaves or television radio waves.
b) Infra–red waves or gamma rays.
c) Ultraviolet waves or infra–red waves.
d) X–rays or visible light.
Check your answers.
Remember–electromagnetic waves do not need a medium to be
transmitted through, however, particular solids, liquids and gases can
absorb particular electromagnetic wavelengths, stopping their
transmission.
The following page displays diagrammatic information on the relative sizes
of different wave types and instructions for eight activities.
You will need coloured pencils to carry out the eight activities indicated in
the diagram.
Part 2: Waves waves waves
11
Information systems
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evidence of infrared waves.
Your body is even emitting
infrared radiation as you
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frequencies in the electromagnetic spectrum are used for different
communication systems.
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EMS and communication
Visible light and infra–red waves
Remember–if one wave passed a point in one second, it would have a
frequency of one hertz. If one thousand waves pass a point in one
second, it would have a frequency of one kilohertz (kHz).
Visible light has a wavelength of 700 to 400 nm.
Infra–red waves have a wavelength of 700 nm to 1 mm.
Visible electromagnetic waves are the colours of the rainbow.
Together, all the colours make up white light. Visible light is used to
scan pages in fax machines. Visible light waves and infra–red light
waves are used to transmit digital information at the speed of light
through optical fibre telephone lines. (Optical fibres will be further
discussed in Part 6 of this module.)
Fax machines, telephones and computer based communication
systems all rely on information transmission through telephone lines.
These communication systems use the visible light and infra–red
sections of the electromagnetic spectrum for communication through
optic fibres.
Bar codes are scanned using visible light. You may have noticed the
red light that is projected onto purchase items at the checkout. This is
visible light used to scan the bar code.
Microwaves
Microwaves are a type of radio wave with a wavelength from 1 mm to
30 cm wavelengths. The type of radio waves mobile phones utilise for
information transmission are microwaves at 824 to 849 megahertz
(MHz). This means 824 to 849 million cycles per second or 824 000 to
849 000 kHz. Land–based telephone systems also use microwave
towers to transmit information over long distances to the next
microwave tower, rather than lay hundreds of kilometres of cables.
Microwaves are used in satellite communication using various
frequencies.
Part 2: Waves waves waves
13
Radio waves
Radio waves have wavelengths ranging between 30 cm and 1 km.
The following information outlines the uses of various wavelengths in
communication.
AM radio waves
AM radio waves are transmitted at 335 kHz to 1.7 MHz (1700 kHz).
These waves carry information on amplitude modulated (AM) waves.
A standard carrier wave, using the frequency allocated to the AM radio
station, is combined with the speech wave from the radio announcer or
music wave from the radio station. The amplitude of the carrier wave is
modified by the speech or music wave from the radio station.
The carrier wave is removed from the radio wave inside a radio receiver
to select only the speech or music frequencies from the radio station.
AM modulated carrier
Notice the amplitude or the height of the wave is modified (modulated)
in the above diagram.
Two–way radios use AM radio waves in much the same way.
FM radio waves
FM radio transmissions occur at frequencies of 88 up to 108 MHz
(88 000 kHz to 108 000 kHz). The waves carry information on a
frequency modulated (FM) wave.
A carrier wave’s frequency is altered with the addition of speech or
music from the radio station. Instead of modifying the amplitude or
size of the carrier wave, it alters the frequency of the wave
transmission. This means the number of waves to pass a point in one
second varies as shown by the following diagram.
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FM modulated carrier
Notice the number of wavelengths to pass a point varies according to
the signal.
FM radio stations use FM radio waves for communication.
Television
Television broadcasting stations transmit their television programs
using at least two FM signals. One signal carries the information for
the television picture and the other carries sound information.
Colour broadcasting uses one FM signal for each phosphor colour on a
television screen, plus an FM sound signal. Sound accompanying a
television broadcast, is transmitted at 5 MHz above the frequency of the
television signal.
Television channels numbers 2–6 transmit at the radio wave frequencies
of 54 MHz (54 000 kHz) to 88 MHz (88 000 kHz) using an FM signal.
Aerial length
Aerial length is about the same order of magnitude as the wavelength
of the electromagnetic waves it is designed to transmit or receive.
The aerial in a mobile phone that receives and transmits microwaves is
only centimetres in length. The metal wire or metal parts in an
aerial/antenna for an FM radio receiver or TV set are closer to a metre
in length. AM radio receivers have many turns of metal wire that can
be hundreds of metres in length in their aerial/antenna. Similarly
transmitting aerials for AM radio are much longer than for FM stations.
Frequency
The table on the following page shows the parts of the electromagnetic
spectrum which are used for communication purposes.
1
Record the frequencies in the frequency column on the electromagnetic
chart on the following page for each communication system from the
text on pages 13–15.
*
2
Cut out the pictures in the Appendix and glue them in the
Communication systems column in the chart on the following page.
Part 2: Waves waves waves
15
Wave frequency
Wave type
Frequency
Communication system
using wave
visible light
infra-red
microwave
FM radio waves
TV radio waves
AM radio waves
Check your answers.
Turn to Exercise 2.2 at the back of this part to identify the types of waves in
the electromagnetic spectrum used in communication.
16
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Microwave and radio wave use
Microwaves are a part of the spectrum of radio waves. When people
refer to radio waves, they are generally referring to the usual AM and
FM radio waves used in radio station and television station
transmissions. Microwaves have a smaller wavelength and a higher
frequency than the general radio waves. It is these properties that make
microwaves more or less useful than general radio waves.
Microwaves
There is little difference between frequency modulated (FM) radio
waves and the frequency modulated microwaves used to send signals
from mobile phones. The only difference is the frequency bandwidth.
Microwave towers can be seen in many parts of inland Australia, on
hills and high buildings in country towns. These microwave
transmission towers have replaced the need to connect distant parts of
Australia by landlines.
The benefits of microwave use in communication are as follows:
•
Microwaves are on a different bandwidth of frequency to radio
waves on the electromagnetic spectrum. Crowding of the radio
wave bandwidths is a problem.
•
Microwaves do not spread out very much so most of the energy
makes it to the next receiver dish from the transmitter. This results
in a signal with the potential range of up to 100 km. Such a system
is important to send information over long distances from tower to
tower on telephone networks, removing the need for cables.
•
It is possible to send a large number of signals at once, because the
range of frequencies in the microwave transmission range is large.
•
Because microwaves have a shorter wavelength, microwaves have
a higher frequency. This means that more information can be
transmitted through microwaves in the same amount of time than
radio waves, which have a lower frequency.
•
Microwaves can also be received and retransmitted by satellites,
expanding the receiving and transmitting area for microwave
communication.
•
Higher frequency waves such as microwaves need less electrical
power for transmission than lower frequency waves such as radio
waves.
Part 2: Waves waves waves
17
Disadvantages of microwave use in communication are as follows:
•
Microwaves travel in straight lines and therefore require a line of
sight connection from one antenna to the next. Because of this, a
mobile network needs a huge number of antennae. Transmitting
and receiving aerials used in remote areas for telephone
transmissions without cables need to be built up to 90 m tall for
line of sight access to towers 50 to 80 km away.
•
Because microwaves travel in straight lines, microwave signals
may be blocked by hills and mountains. This could be the reason
for mobile phone connections dropping out whilst in transit.
•
Microwaves heat food by water molecules within the food
absorbing the waves. This fact explains microwave transmission
difficulties during rain and high humidity as water molecules in the
air tend to absorb the microwaves.
•
Microwave transmission over the ocean is less successful than
transmission over land as water tends to absorb some of the energy.
•
Cell antennas are usually mounted very high on cell towers to have
line of site access over a ten kilometre square area. Interruptions to
line of site transmission by hills and buildings can disrupt
microwave–based conversations.
•
Microwave signals must be relatively strong for information
transfer to occur. The microwave signal is strongest at the cell
tower, losing its strength as it radiates in all directions from the
tower. Towards the outskirts of a cell area, mobile phone
connections tend to break up in clarity or drop out of range. This is
because the microwave signal is not strong enough to be
transformed into electrical impulses by the mobile phone aerial.
Radio waves
Radio waves are beneficial in communication systems for the following
reasons:
18
•
Some radio waves can be transmitted into space and reflected off
satellites. Radio waves are therefore useful for reaching long
distances.
•
AM radio waves, unlike FM radio waves and microwaves, do not
require line of sight access for successful transmission. AM radio
waves can be reflected off objects such as hills, the Earth’s surface
and layers of the atmosphere. This allows AM radio wave
transmission to distant and remote places without the use of
satellites.
•
AM radio waves of high frequency called short waves (SW) can
travel further at night. Atmospheric layers alter their altitude with
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night–fall, allowing radio waves to be reflected over longer
distances. This is often the reason why at night you can tune into
radio channels broadcast from overseas that cannot be detected
during the day.
The disadvantages of radio wave use in communication technologies
are as follows:
•
Radio waves can be absorbed by water, oxygen and carbon dioxide
in the atmosphere, reducing signal intensity.
•
Radio waves can be affected by static produced by passing cars,
overhead power lines and lightning.
•
Low frequency waves such as radio waves need more electrical
power for transmission than higher frequency waves such as
microwaves.
•
Heavy rainfall can absorb radio waves, affecting their transmission.
•
Radio wave transmission over the ocean is less successful than
transmission over land as water tends to absorb some of the energy.
•
AM radio waves are more affected by atmospheric conditions and
frequency ‘noise’ than FM radio waves and microwaves. This
results in static upon reception.
•
Because radio waves can be reflected off objects such as land and
atmospheric layers, the same signal can arrive at a receiver at
slightly different times. This can leave a ghosting effect on
televisions and an echo sound on radios.
Turn to Exercise 2.3 at the back of this part to compare the use of radio
waves and microwaves in communication.
Part 2: Waves waves waves
19
Summary
1
Write three multiple choice questions and their answers based on the
information in this part. Make the questions as challenging as you
would expect in an exam.
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2
Write two short answer questions and their answers based on the
information in this part. Room for questions is also available on
the following page.
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3
Write one long answer question with its answer based on the
information in this part.
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
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Part 2: Waves waves waves
21
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22
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formulas
equations
calculations
particles
energy
interactions
H2O
O H O
H H H
MICRO
O
H H
observe
infer
understand
SYMBOLIC
O H
H
MACRO
Appendix
AM
radio
optic fibre
FM
radio
Part 2: Waves waves waves
23
24
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Suggested answers
What is a wave?
wavelength
1
crest
trough
2
a) The wave frequency is 10 Hz.
b) The wave frequency is 25 Hz.
c) The wave frequency is 300 Hz.
Waves carry energy
1
Electromagnetic waves do not require a medium for transmission.
2
Transverse waves transfer energy perpendicular to an electric field
or particle movement.
3
Longitudinal waves vibrate particles in the same direction as the
flow of energy.
4
Mechanical waves require a medium for wave transmission.
5
Electromagnetic waves can travel through a vacuum because they
do not require a medium for transmission.
6
Mechanical waves cannot travel through a vacuum because they
require a medium for transmission.
Part 2: Waves waves waves
25
The electromagnetic spectrum
1
a) Gamma rays have a shorter wavelength than television radio
waves.
b) X–rays have a shorter wavelength than infra–red waves.
c) Ultraviolet waves have a shorter wavelength than visible light.
d) Infra–red waves have a shorter wavelength than microwaves.
2
a) Microwaves have a higher frequency than television radio
waves.
b) Gamma rays have a higher frequency than infra–red waves.
c) Ultraviolet waves have a higher frequency than infra–red
waves.
d) X–rays have a higher frequency than visible light.
Electromagnetic waves and communication
Wave frequency
Wave type
visible light
Frequency
Communication system
using wave
8 million
megahertz
optic fibre
26
infra-red
4 million
megahertz
microwave
824–849
megahertz
FM radio waves
88–108
megahertz
FM
radio
TV radio waves
54–88
megahertz
television
AM radio waves
535 kilohertz–
1.7 megahertz
AM
radio
mobile
phone
Information systems
Gill Sans Bold
Exercises - Part 2
Exercises 2.1 to 2.3
Name: _________________________________
Exercise 2.1
Identify the following waves as one of the following wave types:
•
electromagnetic transverse wave
•
mechanical transverse wave
•
mechanical longitudinal wave.
You may need to refer to page 6 to help you with your answer.
Wave
Wave classification
microwave travelling through space
wave travelling in the ocean
sound travelling through air
Part 2: Waves waves waves
27
Exercise 2.2
1
2
28
Use the scale on the following page to mark the frequency of the
electromagnetic spectrum of following wave types:
•
microwaves
•
infra–red
•
visible light
•
radio waves used to broadcast
•
TV
•
AM
•
FM .
Next to each wave type, record which of the following
communication systems uses one of the listed frequency ranges
during communication:
•
optic fibre infra red information transfer
•
television
•
AM radio
•
mobile phone
•
optic fibre visible light information transfer
•
FM radio.
Information systems
Gill Sans Bold
Frequency
Wave type
Communication
technology using this
wave frequency
8 000 000 000
kHz
___________________
_____________________
4 000 000 000
kHz
___________________
_____________________
___________________
_____________________
___________________
_____________________
___________________
_____________________
___________________
_____________________
849 000 kHz
824 000 kHz
108 000 kHz
88 000 kHz
54 000 kHz
1 700 kHz
535 kHz
Part 2: Waves waves waves
29
Exercise 2.3
Advantages
Disadvantages
radio waves
microwaves
Wave
Many of the advantages and disadvantages of using radio waves and
microwaves are identical. Use the information on microwaves and
radio waves for communication to identify the benefits and
disadvantages of microwave and radio wave use in communication
systems.
30
Information systems