Senior Science 9.4 Information Systems Section 3 Waves --Properties and Uses 9.4 Section 3 ::: Waves - Properties and Uses 9.4.3 The different waves have different properties which are utilised in a range of communication systems through air and space 9.4.3.a Identify that where information systems cannot be physically linked the information may be transmitted in wave form through the atmosphere or space 9.4.3.b Identify the properties of energy from the electromagnetic spectrum that make it useful in communication technologies including its – speed of travel – ability to travel in a straight line – ability to be reflected 9.4.3.c Describe the individual properties of visible light, radio waves (AM, FM, TV waves) and microwaves and relate these to their use in communication systems 9.4.3.i Plan, choose equipment or resources for, and perform a first-hand investigation to compare communication using AM and FM radio waves © P Wilkinson 2002-04 2 9.4.3.a Identify that where information systems cannot be physically linked the information may be transmitted in wave form through the atmosphere or space Communication with the rest of the world by Australians has undergone tremendous change since the beginning of European settlement. The men and women who arrived at Sydney Cove in 1788 were completely isolated. They could wait for years for a reply to letters from England. Even then, news they received was at least a year old because of the time taken for the voyage. Mail was the main method for long distance communication. Advances in science during the 18th century resulted in a communications revolution. Benjamin Franklin discovered electricity could travel along metal wires. Michael Faraday showed the relationship between electricity and magnetism. These discoveries were used to attempt to send messages along wires. The telegraph was the first device using electricity to send information over a long distance in a short time. The first Australian telegraph line linked Melbourne and Williamstown in 1854. By 1872 Australia could communicate directly with Britain. This required a physical link between the two countries by undersea and overland cables covering a distance of 29 000 kilometres. Building and maintaining physical links such as wires can be very expensive and sometimes not practical and even impossible. In the beginning such links were developed between large cities. People living in small or remote communities or working on ships had no way of joining in the telegraph, and later the telephone, communication networks. The main drawback was the need for cables to carry electricity. The next major change was the development of wireless. This was possible with the scientific discoveries of: James Maxwell who showed mathematically that EM waves could exist. Heinrich Hertz who made the first radio transmitter and demonstrated the existence of radio waves and Guiglielmo Marconi who discovered a way to transmit and receive radio waves. In 1901 he proved that it was possible to send messages from England to Canada using radio waves. An important practical use of the “wireless” was for ship-to-ship and ship-to-shore communication. Radio helped to save thousands of lives of victims of sea disasters. Many uses of radio were soon found, particularly involving moving objects where physical contact was difficult or impossible. By the 1930’s airplane pilots, the police, and military personnel were all using radio for quick communication. The wireless telegraph quickly replaced wire telegraph for long distance communication. A physical link was no longer necessary. It meant communication was now possible to remote areas. It also meant the introduction of a new method of mass communication – the radio station. Radio communication opened the way for many communications technologies: television, satellite communication and mobile phones. © P Wilkinson 2002-04 3 Although short-wave radio provided cheaper international communications and a wide range of services, they were affected by disturbances to the ionosphere in the upper atmosphere. On the other hand cables were unaffected by ionospheric disturbances, and offered continuous service and could be private. In addition, radio capacity was limited but demand for communication services was constantly increasing. Today communication involves a mix of cable and through the air technologies. Optic fibers are replacing copper wire. Microwaves, infrared and light are now being used in communication systems, as well as the traditional radio waves. All are contributing to higher quality and faster communication. Radio waves - a brief history Radio waves were the first electromagnetic waves to be used in modern communication systems. The early radio differed greatly from what we understand as radio today. The signal did not consist of words or music. It was basically a signal (on) or no signal (off). To transmit information Morse code was used: ie a long on signal meant a dash, a short on signal meant a dot; these were spaced by brief periods of no signal. The original waves had relatively low frequencies, long wavelengths (14,300 metres), required large aerials for transmission and reception (1/4 the wavelength of the wave) and needed large amounts of power to generate the electricity needed to create the EM waves. To achieve greater distances it was first thought that more power was needed to send longer wavelength (lower frequency) waves. Scientists discovered in 1923 that short wavelength (90 metres) radio waves could be sent over long distances more easily than waves with a wavelength of 14,300 metres. This new “short-wave” system, known as beam wireless, used directional aerials to concentrate signals, allowing a large reduction in transmitter power. “Short-wave” communication was possible because a particular property of these “short waves”. They could be reflected off the ionosphere, a part of the upper atmosphere. The development of radio technologies resulted in a number of improvements in communication. Beamed radio waves resulted in long distance telegraphy to be much faster. Broadcast radio waves resulted in a new form of mass communication – the radio. This allowed long distance mass communication to also be much faster. Eventually a means of transmitting pictures via radio waves was discovered. Today digital information is now being transmitted by radio waves. © P Wilkinson 2002-04 4 Notes Questions 1. People live and work on places where communication by physical links (eg. wires) is not practical or impossible. Name three such places. 2. Who discovered a way to transmit radio waves? 3. People live and work on places where communication by physical links (eg. wires) is not practical or impossible. Name three such places. Name three locations where rapid and /or long distance communication was made possible by radio. 4. Name one advantage and one disadvantage of radio communication 5. What was the first electromagnetic wave used in modern communication systems? 6. What property of “90 metre short wave” radio waves made them very useful in long distance communication? PRINCIPLE OF THE RADIO © P Wilkinson 2002-04 5 9.4.3.b Identify the properties of energy from the electromagnetic spectrum that make it useful in communication technologies including its – speed of travel – ability to travel in a straight line – ability to be reflected Electromagnetic Waves What is an electromagnetic wave? An electromagnetic wave consists of changing electric and magnetic fields. They are non-mechanical waves, because they do not need a material medium in which to travel. Link to several good Web sights [http://www.vislab.usyd.edu.au/photonics/] [http://edweb.photonics.crc.org.au] Useful properties of EM waves for communication Electromagnetic waves are very useful in communication technologies because of their particular properties. These include: They travel at 300 000 kilometres per second. They travel in straight lines but They spread out from a point source They can be reflected. They can travel through space and do not require a medium for transmission. Radio waves can be emitted and absorbed by metals (therefore transmission aerials and receiving aerials). © P Wilkinson 2002-04 6 General properties of Electromagnetic Waves and related phenomena All electromagnetic waves can be described in terms of their physical properties. properties include amplitude, wavelength, frequency and velocity. These Diagram An electromagnetic wave showing amplitude and wavelength Amplitude Amplitude is the greatest distance of a crest or trough from the equilibrium position ray (wave height). Wavelength Wavelength is the distance between the same points on consecutive waves. Frequency Frequency is the number of times each second that crests pass a stationary point (ie cycles per second). Velocity Electromagnetic waves travel at 300 000 kilometres per second. All waves can exhibit phenomena such as reflection, refraction and diffraction. Reflection occurs when an EM wave “bounces off” a surface. In this case the angle of incidence equals the angle of reflection. Refraction occurs when a wave passes from one medium to another. Refraction causes the wave to bend. It is caused by the wave changing velocity as it changes mediums. Diffraction occurs when a wave passes through a slit or past the edge of an object. Diffraction is the spreading out of the wave at the slit or edge. © P Wilkinson 2002-04 7 Notes Questions 7. What is an electromagnetic wave? 8. How fast do electromagnetic waves travel? 9. The table below contains the information for this question. o Match the property to the general use. o Then write a meaningful sentence using the matched information Property General Use EM waves travel at the speed of light Allowing them to be used in broadcasting or mass communication and therefore reach a large number of receptors at the same time. EM waves travel in straight lines Allowing them to be easily detected and allowing reception of both broadcast and beamed radiation EM waves spread out from a point source Allowing them to be used in (long) distance communication Radio waves can be absorbed by metals Allowing communication into space EM waves do not require a medium in Allowing a beam of radiation to be produced which to travel improving range and therefore used in (long) distance communication EM waves can be reflected Allowing them to bend around objects EM waves can be diffracted The shorter the frequency The longer wavelength Allowing their direction to be changed 10. Radio waves spread out from a point source. This allows them to be broadcast. What is the point source from which radio waves are broadcast? 11. Measure the amplitude and wavelength for the following waves a. b. 12. How many full cycles are shown for each of the four waves drawn above? © P Wilkinson 2002-04 8 9.4.3.c Describe the individual properties of visible light, radio waves (AM, FM, TV waves) and microwaves and relate these to their use in communication systems Frequency range of Communication Systems Today, different bands of radio waves and other electromagnetic waves are used for various communication systems. These include the forms of mass communication – the AM and FM broadcast bands, VHF and UHF television, and the satellite broadcasts (eg Sky channel and Fox). Table Frequency range of various broadcast band waves [Source – Photonics] Frequency Range Wavelength Range Broadcast band or use 500 kHz - 1.6 MHz 600 – 180 meters AM radio 88MHz - 108 MHz 3.5 – 2.5m FM radio [VHF] 174 MHz - 254 MHz 1.7 – 1.2m VHF Television (broadcast) 470 MHz - 860 MHz 64 – 34cm UHF Television (broadcast) 825 MHz - 960 MHz 36 – 31cm Mobile phones 1 GHz - 40 GHz 4 GHz - 8 GHz 12 GHz - 18 GHz 30- 7.5cm (microwaves) Communication satellites C Band (broadcast) Ku band (broadcast) Features of Communication Systems The individual properties of electromagnetic waves and the phenomena associated with these waves help determine their use in communication systems. These properties affect TWO important features of communication systems. These features are: The range of the system The generating power of the transmitter effects the range of the system An object in the path of a wave will result in absorption, reflection, refraction or diffraction of the wave. All of these will affect the range of the system. Focusing waves into a beam will increase the range of a transmission. The amount of information that can be transmitted. The higher the frequency the more information that can be carried by the wave. © P Wilkinson 2002-04 9 In terms of communication it is sometimes easier to identify three sections of the electromagnetic spectrum. These sections are: o Radio waves can be referred to as “radial waves” (round) o Microwaves can be referred to as “linear waves” (straight) o Light (including infra-red) requiring a wave guide Notes Questions 13. Name three forms of mass communication. 14. A radio transmitter produces a signal of 1233kHz. broadcast would it most likely be? At this frequency what type of 15. A radio transmitter produces a signal of 730MHz. At this frequency what type of broadcast would it most likely be? 16. A radio transmitter produces a signal that has a wavelength of 34cm. wavelength what type of broadcast would it most likely be? 17. Which broadcast band has the shortest wavelength? FM radio that is broadcast on the VHF band or VHF television broadcast on the VHF band 18. Which frequency can carry the most information – 600kHz, 600MHz or 6GHz? 19. Name three wave phenomena that affect the range of a wave. © P Wilkinson 2002-04 10 With this AM radio waves – Properties and Uses Property Description / Information AM radio waves have a relatively low 0.5 MHz - 1.6 MHz frequency Radio 2NC transmits on frequency 1 233 000 cycles per second (Hertz – or 1233kHz)) – ie in the MHz (106 or Megahertz) range. Because AM radio waves have relatively low The low frequency of AM radio waves, are frequencies they can only carry small useful to carry voice transmissions amounts of information. (although the quality of the sound broadcast is relatively poor). They carry a single audio signal and this requires only a relatively small bandwidth. The bandwidth of such radio waves is 10 kHz. Long wavelength Wavelength varies from about 600 - 180 metres. AM radio waves diffract (bend around objects) with a relatively small loss of energy. This property of AM radio waves is due to their long wavelength. This means that a mountain will not necessarily stop AM radio signals. It also means that these signals will travel along the curved surface of the earth (they bend or diffract along the curved surface). This long wavelength and diffraction of AM radio waves gives them a long range and therefore they are very useful when broadcast radio waves are needed. © P Wilkinson 2002-04 11 Reflection from a layer in the atmosphere, the This enables radio signals to be transmitted ionosphere (and the ground) to receivers which are “out of sight” because of the Earth’s curvature. This gives AM radio a very long range. Signals can be received thousands of kilometres away. Therefore reflection off the ionosphere makes them useful in long distance communication. Like all EM waves, AM radio waves travel in straight lines. Using directional antennas these waves can be beamed from one location to another with a certain degree of accuracy. Beamed wireless requires directional aerials to concentrate signals, allowing large reductions in transmitter power. Beaming of waves that travel in straight lines is useful in long distance communication. AM radio waves can travel through almost Therefore, they can be used to any medium. communicate from under water, from within buildings and deep inside mines. The advantage of AM radio is that the circuitry required is not overly complicated. Therefore it is cheap and readily available for a large variety of uses. As a consequence the AM band was the first to be developed and is relatively crowded with users. One problem with AM radio waves is they are susceptible to interference. Atmospheric conditions (storms, particularly electrical storms) can severely affect electrical equipment and therefore the quality of the transmission. The AM radio band has a large variety of uses. Originally it was very important in radio telegraphs and shore-to ship and ship-to-ship communication. Many uses have been found for AM radio – with airplanes, police vehicles, Radio Broadcasting, two way (walkie-talkie) communication and CB radio. © P Wilkinson 2002-04 12 Notes Questions 20. What is meant by the term MHz? 21. Identify the type of information carried by AM radio. 22. What useful property of waves allows AM radio waves to reach receivers that are out of “line of sight” (such as behind a mountain)? 23. What property of radio waves allows them to be broadcast? 24. What layer in the atmosphere can reflect radio waves? 25. Name four uses of radio waves. 26. Why is AM radio relatively cheap? 27. What natural events can affect the quality of transmission of AM radio waves? 28. Complete the table below listing all the properties of AM radio waves and any wave phenomena they exhibit. [Use all the information in this chapter to answer this question] Properties of AM radio waves © P Wilkinson 2002-04 Phenomena exhibited by radio waves 13 FM Radio waves (VHF) Property Description / Information FM radio waves have frequency that is much 88MHz - 108 MHz higher than AM radio waves. (VHF stands for Radio JJJ transmits on a frequency of 102.1 Very High Frequency) million cycles a second (or 102.1MHz). This puts them in the VHF radio band. The total bandwidth required to carry FM FM radio waves can carry more information signals is about 200kHz. The increased than AM radio waves. The property that bandwidth is required to transmit high quality allows this information to be carried is the audio signals (stereo). Therefore, FM radio higher frequency of FM waves. waves can be used to broadcast stereo sound. Because of this higher quality and the crowding of the AM band, commercial radio has gradually shifted to this band. The wavelength of FM radio varies from about 3.5 – 2.5 metres Because FM waves have shorter wavelengths than AM waves they do not diffract as well around large objects (ie bend around objects). The diffracted wave loses too much energy. Therefore, FM radio signals will bend partially around mountains and tall buildings but these objects can significantly reduce signal strength. This results in poor reception for some TV signals in hilly areas (ie not line of sight). FM radio waves spread out from a point This allows FM to be broadcast. source. This means the signal can reach a FM radio waves do not reflect off the large number of receivers at the same time. ionosphere. The result is that FM waves cannot be received further than the horizon as seen from the antenna (line-of-sight). The range can be increased by using a powerful transmitter or by using relay transmitters. This allows them to be used for broadcast radio and marine radio frequencies. The circuitry required to transmit FM radio waves is much more complicated than AM circuitry. Notes Questions 29. What property of waves allows FM radio to carry more information than AM radio? 30. What can make reception of FM broadcast signals difficult in some areas? 31. The ionosphere does not reflect FM radio waves. How does this affect their range? © P Wilkinson 2002-04 14 TV radio waves – (VHF and) UHF VHF waves used for TV broadcasts have a frequency range of 54 MHz - 88 MHz and 174 to 254 MHz. The wavelength of VHF radio waves 5.6 metres – 1.2 metres. UHF waves used for TV broadcasts have a frequency range of 470MHz – 860 MHz. The wavelength varies from about 65 centimeters to 30 centimeters. Channel 3 in Newcastle used to transmit at a frequency of 91.0 million cycles a second (or 91 MHz) and is therefore in the VHF range. It can be detected on a FM radio. Since 20____ it has transmitted on the UHF band – at _____ MHz. Most TV stations in Australia have moved to the UHF band. Both VHF and UHF waves spread out from a point source (ie the aerial). This allows them to be broadcast from transmitting aerials. Like all electromagnetic waves VHF and UHF travel at the speed of light. This allows for very fast communication. Tall buildings and hills can cause major problems with TV reception of both VHF and UHF. Diffraction of the VHF signal occurs but the diffracted signal is very weak. Weak signals also occur because buildings and hills absorb the waves. TV reception of the original signal and a reflected signal causes ghosting. This is a bigger problem with VHF signals. Problems such as these can be overcome using relay stations in country areas, or for a large city a multiple number of Broadcast aerials. Like all EM waves UHF radio waves travel in straight lines. Therefore UHF broadcasts are limited to line of sight paths. Broadcast antennas are usually mounted at the top of a high mountain to allow a maximum broadcast range [about 80 kilometers]. To ensure good reception, aerials are mounted high up on roofs. The range is however adequate for TV Broadcasts. © P Wilkinson 2002-04 15 The high frequency of VHF and UHF is the property that allows a lot of information to be carried. TV waves must carry more information than FM waves. The total bandwidth required to carry TV signals is about 6MHz. There needs to be separate signals sent for each of the three colours (blue, yellow and red) as well as sound information (stereo sound) and other information to control the picture. Obviously UHF can carry more information than VHF because of its higher frequency, allowing higher quality transmissions. The short wavelength of UHF means that reflection can be a useful property. In a crowded urban area, with buildings close together, these reflections can cause rapid variations in signal strength. Reception may suddenly become clear, as a particular reflection path becomes dominant. A small change in the position of the antenna may cause a major change in reception quality because a strong reflection can now be detected. Such reflections are frequently useful, as they may allow communications between two stations over a highly obstructed path. This situation is obvious to users of mobile phones. VHF and UHF have a number of uses. As technology has improved many communication systems are moving to these bands. They give higher quality transmissions and are not affected as much by atmospheric conditions (storms). VHF is used for FM radio, some TV Broadcasts and marine radio. UHF is used for TV broadcasts, police radio and there are a number of amateur radio bands. As well, the high end of UHF (very close to the microwave band) is used for mobile phones. Notes Questions 32. Which has the higher frequency – VHF or UHF? Answer using examples. 33. Research Find out the frequency of a local TV station. 34. How fast do UHF waves travel – in kilometers per hour? 35. What property of waves allows VHF to be broadcast? 36. Explain how diffraction limits the range of UHF TV waves. 37. What problem is caused by the reflection of VHF TV waves? 38. Outline the advantage that can result from the reflection of UHF waves? 39. Name two communication systems that use UHF waves. 40. Which signal carries more information – VHF or UHF? What property is important © P Wilkinson 2002-04 16 What to do 1. Summarize the information on TV waves using a mind map. In the mind map identify three uses of TV waves. List the properties of TV radio waves that relate to their use. The mind map has been started for you. Speed of light travel –very fast communication Straight line – must be line of sight but reflections useful Other use Mobile phone Spread out from pt source to reach mobile phone tower TV waves Other use © P Wilkinson 2002-04 Small wavelength small aerial & small diffraction 17 Microwaves A microwave is a very short radio wave. Microwaves have a frequency range of 300 to 1 thousand million cycles a second (Hz). The wavelength of a microwave varies from about 30 centimeters – 1 millimetre. Microwaves travel in straight lines. Using a microwave dish they can be reflected, and like light waves, can concentrated into beams. The range of low powered, broadcast microwaves is quite small. Microwaves have a small wavelength and therefore they do not diffract (bend) around obstacles. Also, metallic objects will absorb microwaves and therefore they do not pass through such objects. Most large objects will interfere with transmission of microwaves. Therefore, devices using microwaves must have line of sight between transmitter and receiver to operate effectively. The high frequency of microwaves gives them large carrying capacity. Microwaves are called broadband links because they carry a very large volume of communications. A single band can carry thousands of telephone calls at the same time. Microwaves are used in telephone and television communication to relay high volumes of information over long distances. The microwaves are relayed along a series of towers set up at about every forty kilometres. Each tower must be within sight of the top of the towers on either side of it, and there must be no intervening obstacles. For more information look up: http://www.telstra.com.au/classroom/sec_4_4.htm Microwaves beams can pass easily through rain, smoke, and fog and they also pass through the ionosphere & travel through space. Thus microwaves are well suited to satellite, and space communications. Since microwaves travel in straight lines, a correctly shaped satellite dish can beam a signal to a defined footprint on the earth’s surface. An important use of the reflection of microwaves is RADAR. Radar systems compare transmitted and reflected radio signals to find the distance of an object from the transmitter, and the object’s relative speed to the transmitter. Notes Questions 41. Which has the higher frequency – microwaves or UHF? Answer using examples. 42. What property of microwaves allows them to be concentrated into beams? 43. Explain why there must be “line of sight” between transmitter and receiver when transmitting microwaves. 44. What is the advantage of microwaves having a high frequency? © P Wilkinson 2002-04 18 Light waves – Infra red “light” and visible light Light has been used in communication for thousands of years. The Greek victory in the Trojan War was telegraphed by lighting hilltop bonfires one after the other. Heliographs developed by the Romans were another communication using light. Most old communication systems using light had a number of limitations. They carried a limited amount of information; they were slow, unusable in bad weather and labour intensive. Multiple observers, all within sight of each other, had to take down the message and repeat it along the chain. Today a new method of communicating by light has been developed. Optical fibres are a key element in light wave communication. The main job of an optical fibre is to guide light with a minimum loss of the signal. Optical fibres are fine threads of glass that are capable of transmitting light at about 2/3 the speed of light in a vacuum. The properties of light relevant to optical fibres and allows light to travel through the fibre are: light travels in a straight line Light is reflected inside the fibre Light is refracted In addition The very high frequency of light waves gives them huge carrying capacity To be utilised in communications LASER light is used inside an optic fibre. Light will not pass through opaque materials. It is readily absorbed by natural surroundings. For this reason it is not a good free-to-air broadcast medium. With these properties light is useful in carrying the large volumes of data in telephone systems and TV systems (cable TV). Notes Questions 45. Which has the higher frequency – infrared or visible light? Answer using examples. 46. What material is used to make an optic fibre? 47. What is the function of an optic fibre? 48. How fast does light travel in air? 49. How fast does light travel in an optic fibre? 50. What properties of waves keep the light inside the fibre? © P Wilkinson 2002-04 19 9.4.3.i Plan, choose equipment or resources for, and perform a first-hand investigation to compare communication using AM and FM radio waves COMPARING AM and FM RADIO AIM To Compare communication using AM and FM radio waves Information Equipment The equipment needed to make measured comparisons between AM & FM radio waves are expensive and require expertise. Therefore, qualitative comparisons should be made. Method [VGMANS] a. Independent variable is the type of signal b. The dependent variable is sound quality and volume. c. To compare AM and FM a number of variables will need to be controlled. Power of transmitter Distance to transmitter Quality of radio receiver – How? Places to compare include effect of obstacles buildings, hills, and other effects - powerlines, aircraft, moving receiver (car), underground (car park) d. Two Groups are compared – AM, FM e. Qualitative Measurement of sound quality and volume. f. The activity is a practical investigation. What to do 1. Write a heading 2. Copy the problem into your book 3. Discuss a possible method to perform a first hand investigation of the problem. The questions below might help. a. Which radio stations would be the best to compare? Is there a radio station that transmits both AM & FM in your area? Where are the transmission aerials? b. It might be useful to listen to some stations before the method is written. c. Make a draft list of instructions needed to set up this experiment. Discuss your ideas. 4. Write a possible method. It should be proof read by at least one other person. 5. Perform the investigation. 6. Record the results by describing how the quality of the music and voice on AM and FM compared. 7. Write a conclusion. 8. Complete the discussion questions g. It is important to have a large Number of “subjects” to even out individual interpretation of quality and volume. This is to ensure reliable results. © P Wilkinson 2002-04 20 Marking Criteria for method Method can be repeated and Information collected from at least two sources and Both first hand and secondary sources used Method can be repeated and Information collected from at least two sources and/or Both first hand and secondary sources used Marks 2–3 1–2 Discussion questions Syllabus Outcome – H11 -11.2 a,b,c,d, -11.3 a,b; H12 -12.2 b; 1. The dependent variable in this investigation is the type of radio signal – AM or FM. Identify the independent variable? [1 mark] 2. Name TWO variables that need to be kept constant? [2 marks] 3. Name the TWO groups in the investigation. [2 marks] 4. Describe how variables controlled so that the two groups can be compared. 5. Identify what is measured in this investigation. [1 mark] 6. Explain why the activity should be repeatable. [2 marks] 7. Outline how this investigation is reliable. [3 marks] [2 marks] 8. Identify TWO safety issues in this investigation. [2 marks] High band question 9. Discuss the validity of the data collected in this investigation. [5 marks] Discuss (and evaluate) questions are unstructured and require an extended answer In order to answer such a question, you must provide some structure. STEP 1 Identify (& highlight) the important words in the question STEP 2 Recall definitions of these important words (if necessary) Discuss – identify issues and provide points for and against Validity – STEP 3 Develop your own answer that reflects the depth required (Verb & marks) © P Wilkinson 2002-04 21 Marking Criteria for method Identify at least two issues that relate to validity Provide detailed points for and/or against at least two issue Identify one or two issues that relate to validity Provide points for and/or against one issue Outline or identify some points for and/or against related to validity © P Wilkinson 2002-04 22 Marks 5 3-4 1–2 Principle of the Radio To communicate by radio, you need: A radio receiver A radio transmitter Transmission Sound waves are converted into electrical signals in the microphone (voice). An electrical signal of radio wave frequency is produced in another circuit (carrier). These two signals are combined: this is known as modulation. The resulting signal is passed to the transmitting aerial and is emitted as an electromagnetic radio wave. Reception The EM wave is picked up (produces an electric current) by the antenna of a radio receiver. The carrier frequency is separated from the voice signals by a detector in the radio receiver: this is called demodulation. The loudspeaker of the radio converts the electrical signal back to sound. Broadcast or beamed radio AM or FM The generating power of the transmitter effects the range of the system As a general rule the higher the frequency the shorter the range of the transmitted wave (for radio waves) An object in the path of a wave, will result in Absorption of the wave Reflection of the wave or The wave will bend (diffraction) around the object. Low frequency, long wavelength waves can bend around objects (diffraction). For this reason they tend to be absorbed less, giving them greater range. High frequency, shorter wavelength radio waves cannot bend around objects easily and are therefore absorbed or reflected. This means they have a shorter range. The amount of information that needs to be transmitted. The higher the frequency the more information that can be carried by the wave. The bandwidth of a signal is the range of frequencies required to represent the information contained in the signal: it is expressed in Hertz or Hz. The bandwidth required depends on the type of signal. Since EM waves travel at the “speed of light” they are very useful for high-speed long distance communication. Because EM waves travel in straight lines, a beam of radiation can be produced. This improves the range of the radiation and lowers the transmitting power needed. The fact that EM waves spread out from a point source makes them useful for broadcasting and mass communication. It means that large numbers of people can receive the same signal at the same time. The absorption of EM waves by metals allows them to be easily detected. This is obviously important for receiving both broadcast and © P Wilkinson 2002-04 23 beamed transmissions. Communication into space is possible because EM waves do not require a medium in which to travel. With the space age this it is very important for scientists to communicate with a large variety of satellites Communication, CB radio. AM radio waves have a relatively low frequency (0.5 MHz - 1.6 MHz). Radio 2NC transmits on frequency 1 233 000 cycles per second (Hertz – or 1233kHz)) – ie in the MHz (106 or Megahertz) range. AM radio waves have long wavelengths that vary from about 600 - 180 metres. An important property of AM radio waves is they can be reflected from a layer of the atmosphere, called the ionosphere. This enables radio signals to be transmitted to receivers which are “out of sight” because of the Earth’s curvature. This gives “short waves” a very long range. A powerful transmitter (50 000 watts) can result in a signal being received thousands of kilometres away. Therefore reflection off the ionosphere makes them useful in long distance communication. Like all EM waves, AM radio waves travel in straight lines. Using directional antennas these waves can be beamed from one location to another with a certain degree of accuracy. Beamed wireless requires directional aerials to concentrate signals, allowing large reductions in transmitter power. Beaming of waves that travel in straight lines is useful in long distance communication. A property of AM radio waves is their long wavelength. This long wavelength allows AM radio waves to diffract (bend around objects) with a relatively small loss of energy. This means that a mountain will not necessarily stop AM radio signals. It also means that these signals will travel along the curved surface of the earth (they bend or diffract along the curved surface). This long wavelength and diffraction of AM radio waves gives them a long range and therefore they are very useful when broadcast radio waves are needed. AM radio waves can travel through almost any medium. Therefore, they can be used to communicate from under water, from within buildings and deep inside mines. AM radio waves have relatively low frequencies and therefore can only carry small amounts of information. They carry a single audio signal and this requires only a relatively small bandwidth. The bandwidth of such radio waves is 10 kHz. This low frequency means AM radio waves are useful to carry voice transmissions (although the quality of the sound broadcast is relatively poor). The advantage of AM radio is that the circuitry required is not overly complicated. Therefore it is cheap and readily available for a large variety of uses. As a consequence the AM band is relatively crowded with users. One problem with AM radio waves is they are susceptible to interference. conditions (storms) can severely affect the quality of the transmission. © P Wilkinson 2002-04 24 Atmospheric FM FM radio waves have a relatively low frequency (88MHz - 108 MHz). Radio JJJ transmits on a frequency of 102.1 million cycles a second (or 102.1MHz). This puts them in the VHF radio band. The wavelength of FM radio waves varies from about 3.5 – 2.5 metres. FM radio waves travel in straight lines but they spread out from a point source. FM radio waves do not reflect off the ionosphere. The result is that FM waves cannot be received further than the horizon as seen from the antenna (line-of-sight). A powerful 100000-watt FM station can broadcast up to 100 kilometres. This range makes them suitable for a number of uses - Broadcast radio and Marine radio frequencies. Because FM waves have shorter wavelengths than AM waves they do not diffract as well (ie bend around objects). Therefore, FM radio signals will bend partially around mountains and tall buildings but these objects can significantly reduce signal strength. They are also absorbed more readily than AM radio waves, which also affects range. FM radio waves must carry more information than AM radio waves. The total bandwidth required to carry FM signals is about 200kHz. The increased bandwidth is required to transmit high quality audio signals (stereo). The property that allows this information to be carried is the higher frequency. Therefore, FM radio waves can be used to broadcast stereo sound. Because of this higher quality and the crowding of the AM band, commercial radio has gradually shifted to this band. RADIO WAVES – PROPERTIES AND USES What to do Read the information below on the properties and uses of EM waves. 2. Summarize the information in a table (for each of the bands) Include all EM communication bands List as many uses and properties related to those uses as possible. Communication band Use ________________________ Properties Communication band Use ________________________ Properties 3. Write an information report on the properties of individual EM waves and relate these to their use in communication systems. An information report scaffold is shown below. © P Wilkinson 2002-04 25 Introductory paragraph Concluding paragraph © P Wilkinson 2002-04 26 © P Wilkinson 2002-04 27