Senior Science

9.4 Information Systems

Section 2

Transmitting Waves

© P Wilkinson 2002-04

9.4 Information Systems – Section 2

9.4.2 Information can be transmitted in the form of waves

9.4.2.a Identify the types of waves in the electromagnetic spectrum currently used for communication systems as

visible light infra-red microwaves radio waves, which include:

TV FM radio waves AM radio waves

9.4.2.b Compare the advantages and disadvantages of using microwaves and radiowaves in communication technologies

9.4.2.c Identify communication technologies that use energies from the electromagnetic spectrum for communication purposes

9.4.2.i Perform a first-hand investigation to observe ways in which waves can be modulated to carry different types of information

© P Wilkinson 2002-04 2

9.4.2.a Identify the types of waves in the electromagnetic spectrum currently used for communication systems as visible light, infra-red, microwaves, and radio waves (which include TV, FM radio waves and AM radio waves)

The Electromagnetic Spectrum

It is clear water waves carry large amounts of energy. During large storms tonnes of sand can be shifted by waves. Surfers use the energy from waves to ride the surf. Tidal waves created by earthquakes can do enormous damage. These water waves carry energy from one place to another.

Water waves are visible. There are other waves that transfer energy from one place to another that cannot be seen. People hear things – the sound moves through the air as a sound wave. Sound waves are produced by (and also produce) objects that vibrate.

The sun is the main source of energy for life on earth. The sun radiates various types of invisible energy, such as UV and microwaves. People can detect some types of radiant energy – light is detected by the eyes and infrared is detected by heat receptors in the skin. Again the energy reaches the earth by waves. These waves belong to a large spectrum of invisible radiations called the electromagnetic spectrum. The spectrum, in order from the waves with lowest energy to those with the highest energy, starts with radio waves, then microwaves, infra red, light, ultra violet, X-rays and gamma rays.

The electromagnetic spectrum is very important in many communication systems. Communication is the process of transmitting and receiving information in the form of signals. High technology communications use electromagnetic waves and pulses in varying patterns to send and receive information. Some of the most dramatic changes in modern society have resulted from the ability to send messages and images thousands of kilometres away without any time delay. International football matches and political upheavals become world events as they can be seen by millions of people at the same time, through the use of the electromagnetic spectrum.

The types of waves from the electromagnetic spectrum currently used for communication systems are:

visible light infra-red microwaves radio waves, which include:

TV FM radio waves AM radio waves

© P Wilkinson 2002-04 3

The Parts of the Electromagnetic Spectrum

The diagram shows the parts of the electromagnetic spectrum. The radio wave section has been expanded.

__ _____ _______ _

Band MF HF VHF UHF SHFFrequency 300kHz 3MHz 30MHz 300MHz 3GHz 30GHz

Wavelength 1km 100m 10m 1m 10cm 1mm

microwaves

Radio waves

© P Wilkinson 2002-04 4

AM Broadcast Band

27MHz CB/Marine RadioVHF - TV

VHF – FM Broadcast Radio

VHF Marine Radio

UHF TV

Mobile Phones

Domestic Microwaves

Red >> Blue

< Light > Gamma rays ------

Radio Infra red UV X-rays ---------

1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1020 1021Hz

© P Wilkinson 2002-04 5

Diagram Wavelength, Frequency, Amplitude

Wavelength is the distance between the same two points on consecutive waves.

The number of complete cycles (wavelengths) that pass a point each second is called the frequency of the wave

The amplitude (wave height) is the maximum displacement from the equilibrium.

© P Wilkinson 2002-04 6

Notes Questions

NB All questions are to be answered in meaningful sentences

1. Name two types of waves that are not part of the electromagnetic spectrum.

2. Name two types of waves from the electromagnetic spectrum currently used for communication

3. Name two types of waves from the electromagnetic spectrum that are not currently used for communication

4. Which electromagnetic wave has the lowest energy?

5. Does infra red radiation carry more energy than microwaves?

6. Research a. Which electromagnetic waves have enough energy to kill?b. What problem is linked to UV radiation?

7. Radio waves have a number of uses. Outline one function of radio waves.

8. According to the diagram do mobile phones use microwaves?

Wavelength refers to the distance from one wave to the next. This distance varies from several kilometers to a tiny fraction of a millimeter.

Frequency refers to the number of waves that pass a point each second. Frequency varies from several thousand cycles per second to many billions of cycles per second.

9. How long are AM Broadcast Band radio waves (What is their wavelength?)?

10.How long are microwaves?

11.Which electromagnetic wave has the shortest wavelength?

12.Complete the statement indicating the relationship between frequency and wavelength:As wavelength decreases frequency ……………………

13.Which electromagnetic wave has the shortest frequency?

14.SHF means Super High Frequency. What is meant by the terms – UHF, VHF, HF?

15.Name one organization that broadcasts on VHF.

16.Name one organization that broadcasts on UHF.

kHz (kilo hertz- 103) means 1000 cycles a second

MHz means (Mega hertz - 106) 1000 000 cycles a second

GHz means (Giga hertz – 109) 1000 000 000 cycles a second

17.What is the wavelength of a 300kHz radio wave?

18.What is the frequency of a 1mm wave?

19.Which has the smallest frequency – red or blue light?

20.What is the frequency range of infra red radiation?

© P Wilkinson 2002-04 7

9.4.2.b Compare the advantages and disadvantages of using microwaves and radio waves in communication technologies

Comparing Microwaves and Radio waves

Author’s note The syllabus statement is ambiguous Does it mean to compare the advantages and disadvantages of microwaves to the

advantages and disadvantages of radio waves OR Does it mean the advantages and disadvantages of microwaves and radio waves

compared to other forms of communications

In the high-tech world of long and short distant communication, rapid and high volume communication can be achieved by using electromagnetic energy in two ways:

1. Through the air systems – Electromagnetic waves are sent through the atmosphere or through empty space. Examples include radio, TV, radar, garage door openers, remote TV controllers, pagers, traffic lights, cordless telephones, and lighted signs.

2. Through cable systems – Electromagnetic waves are sent through a wave-guide or conductor such as fiber optics cable or metal cable. Examples include fiber optics or copper wired telephones, wire connected computer elements, intercoms and microphones.

In practice, many modern communications systems use both methods in transmitting a single signal.

Through the air systems such as microwaves and radio waves have advantages and disadvantages when used in communication technologies.

[Source – How the New Technology Works: A Guide to High Tech Concepts]

Advantages

1. The signal and receiver do not need to be physically connected to each other. Therefore, Communication is possible with remote sites. Communication is possible while moving rapidly.

2. Signals can be sent (broadcast) to a large number of receivers at the same time. This makes rapid dissemination of emergency messages.

3. It’s possible to move transmitters (and receivers) without laying new connecting lines.4. It’s possible to reach an individual whose whereabouts is unknown. Pagers enable

organisations never to be “out of touch” with “key” people.5. It is helpful in the study and protection of wildlife. By placing a small radio transmitter on

an animal, information about the life cycle can be learnt without being intrusive.

© P Wilkinson 2002-04 8

Limitations

1. Because there is a limited number of broadcast frequencies, the use of transmitters must be restricted in some way or they will seriously interfere with each other.

2. Through the air signals can be distorted by atmospheric conditions, and signals from space are sometimes distorted by solar activity.

3. Physical obstacles such as mountains and buildings can prevent or interfere with some transmissions.

4. Through the air signals are not private; any correctly tuned receiver within their range can pick them up. This is a significant problem with cordless and cellular phones.

Notes Questions

21.Name three communication technologies that use electromagnetic waves transmitted through the air.

22. Name three communication technologies that use electromagnetic waves transmitted through cable systems.

23.Explain why ships need “through the air” communication systems rather than cable systems.

24. Identify one advantage of through the air systems.

25.Outline one disadvantage of through the air systems.

© P Wilkinson 2002-04 9

9.4.2.c Identify communication technologies that use energies from the electromagnetic spectrum for communication purposes

Communication Technologies Using EM Radiation

What to do

1. Read the following information found in PULSE winter 2000 [A publication of Sony]2. Identify the communication technology and the type of EM wave used in this technology.

Laser link is yet another new facility offered on selected Sony Handycam models. It allows you to watch your video recordings on TV without the need for cables, by using an optional infrared receiver (IFT-R20). This wireless transmission of audio and video data from the Handycam directly to the TV works by using a line of sight infra red transmission, with a range of up to 8 metres. Laser link makes viewing your recorded footage as easy as watching TV. Just point your Handycam at the infra red receiver and press play.

3. Copy the table below and enter the information given above.4. Complete the table with at least 10 communication technologies that use energies from

the EM spectrum [Technologies include fibre optic cable, checkout scanner at the Supermarket, fax machine].

Communication technology EM wave(s) used in this technology

Traffic lights Visible light

© P Wilkinson 2002-04 10

9.4.2.ii Perform a first-hand investigation to observe ways in which waves can be modulated to carry different types of information

Principle of the Radio

Information can be transmitted in the form of waves.

In the radio telegraph the signal was coded using Morse Code – a series of short (dots) and long (dashes) signals. To do this the radio transmission was switched on and off. Short or long bursts of radio energy produced the dots and dashes of Morse Code used in the telegraph. In this case the coding device was a tapping key.

A lighthouse uses another form of electromagnetic radiation, light energy (waves), to signal danger.

The radio was the first technology developed to transmit data by modifying waves.

Question How can waves be modified to carry information?Answer: The waves are modulated. Modulation means to change the amplitude or

frequency of a wave by using a lower order frequency.

There are two kinds of modulation: Amplitude Modulation (AM). The amplitude of the radio wave is increased or

decreased according to the amplitude of the audio signal.

Frequency Modulation (FM). The frequency of the radio wave is increased or decreased according to the frequency of the audio signal. This is done at a higher frequency than amplitude modulation. FM is more difficult to picture than AM, but the principle is similar to AM.

© P Wilkinson 2002-04 11

The Radio Transmitter – The Coding device

In the AM radio transmitter:

One electric circuit produces the audio signal (speech, music). Sound waves are converted into electrical signals in the microphone. This signal:- has a low, varying frequency (20 – 20000 Hz)- has an amplitude (height) that changes.(- and is a direct current)

Another circuit creates a carrier signal. An electrical signal of radio wave frequency is produced. This signal:- has a high, constant frequency

(eg 2NC – 1 233 000 Hz, or 1233 kHz) and- has a constant amplitude (height)(- and is an alternating current)

The low frequency audio signal cannot produce an electromagnetic wave but can cause a loudspeaker to operate to produce sound waves that travel short distances.

The high frequency carrier signal cannot cause a loudspeaker to operate but can produce an electromagnetic wave that travels long distances

A third circuit creates a modulated signal – ie the two signals are “added” together; that is the audio signal modulates the carrier signal in a modulating circuit. This signal:- has a high, varying frequency- has an amplitude that changes(- and is an alternating current)This travels to the transmitting aerial and is emitted as a modulated electromagnetic radio wave. This modulated wave carries the audio signal long distances.

The Radio Receiver – The Decoding Device

The antenna, of the radio receiver, detects the modulated radio wave. A modulated electric current is produced in the antenna.

The function of the receiver is to demodulate the modulated signal. This means it separates the audio signal from the carrier signal.

© P Wilkinson 2002-04 12

The low frequency audio signal can cause a loudspeaker to operate to produce sound waves that travel short distances.

© P Wilkinson 2002-04 13

Notes Questions

26.Name the code used in telegraph communication.

27.Name the two kinds of modulation.

28.Which is transmitted at the highest frequency – AM or FM?

29.List the differences between the audio signal and the carrier signal.

30.List the similarities between the carrier signal and the modulated signal.

31.What is the function of the carrier signal?

32.Use a table to answer the following. List the similarities and differences between the modulated electrical signal and the modulated radio signal.

33.Draw an amplitude modulated signal.

34.Draw an amplitude-modulated, high frequency wave produced by the following audio signals and carrier waves.

a.

Audio signal + Carrier signal = Modulated signal

b.

Audio signal + Carrier signal = Modulated signal

35.What happens when the radio wave is demodulated?

© P Wilkinson 2002-04 14

Observing waveforms using an oscilloscope

Oscilloscope

What to do

Use an oscilloscope to observe various electrical currents.o Currents produced by a tuning fork placed in front of a microphone.o Currents produced by a speaking into a microphone – an audio signal.o Currents produced by AM radio stations an amplitude modulated signal.

Results

Draw your observationsOutline the difference between the two signals.

Light telegraph

A message can be sent simply by switching an electromagnetic wave on and off. This is the principle of Morse code.

What to do

1. Set up the following equipment.

© P Wilkinson 2002-04 15

2. Using Morse code, make up a 3 or 4 word message.Send the message to someone else using the Light telegraph.

© P Wilkinson 2002-04 16