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Electromagnetic Spectrum

The history and uses of the different sections of the EM spectrum as used in communication technologies by society.

h d

on 29 July 2010

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Transcript of Electromagnetic Spectrum

The World
WAVES CDs are an important invention in society today that allows communication through the storage of data. They can be used to hold music, computer software, documents and other forms of data, because of their reliability, availability, low cost and efficiency they have become the standard medium for the distribution of larger amounts of information, for example computer software.
When this technology was released in the early 1980s it was initially used specifically to store audio, however reasearch of laser and optical disc technology allowed advances to be made in this technology and the storage of data such as microsoft word documents and images was made possible.
By usign eight-to-fourteen modulation (EFM) longer playing time and a higher resilience against scratches and fingerprints, which caused defects in the cds, was standardized in this technology.
However CDs are not fully protected from these defects so if not taken care of properly a CD may not be as efficient for transferring data (in particular, smaller amounts of data such as music) as other technologies such as the internet or bluetooth would be.
CDs have lots of tiny bumps on them, these indentations are reffered to as 'pits' and the spaces between these pits are refered to as 'lands'. Each pit is about 100nm deep and 500 nm wide, it's length varies from 850nm to 3.5µm. And the distance between each bit is 1.6µm. CDs use infrared technology, to make use of a CD a CD player or similar device is needed. the job of such device is to find and read the data that is stored in the pits on the CD. The CD must be spun by the drive motor in this device, as the information is stored in a spiral formation around the CD. A laser and a lens system are used to focus on the pits and read them and a tracking mechanism will move the laser so that its beam can follow the pits on the spiral track.
This laser beam passes through the polycarbonate layer on the CD and reflects off the aluminum layer, hitting an opto-electron device which detects changes in light- this works effectively as the pits reflect the light differently to the lands. The electronics in the CD player, or the device reading the CD then detects and interprets these changes in light in order to read the information.
The most difficult part to this device is keeping the laser beam in position on the track, which is why a tracking system is a part of the device. As a CD is played the tracking systems must continually move the laser outward, and as the laser moves outward form the centre of the CD the pits move past the laser faster. This is due to the linear speed of the pits, which is equal to the radius times the speed at which the disc is revolving. So a spindle motor must slow the speed of the CD down as the laser moves outwards and the pits will be travelling past the laser at a constant speed- and the data will be read from the CD at a constant rate. Laser stands for light amplification by stimulated emission of radiation, it is a mechanism that is used for emitting electromagnetic radiation (generally light/visible light) using a process called stimulated emission.
Stimulated emission is photon-related electron transition in a atoms. If an electron has a higher energy level and a photon with an energy level that is equal to the difference between the electron's energy and a lower energy, the photon will stimulate the electron in a way that it will fall into the lower energy state, emitting a photon. The emitted photon will have the same energy as the first photon- so as waves they will both have the same frequency, they will interfere, creating a more intense wave. So in lasers, atoms are kept in an excited state, where some photons are inserted. Some of the atoms will undergo stimulated emission, resulting in more photons which will cause other atoms to undergo stimulated emission- the overall result is a chain reaction and a very intense and coherent light.
Lasers can be manipulated with a lense and are typically of narrow wavelength, although it is possible for some lasers to emit a borader spectrum of light as well as emit different wavenlegths of light at the same time.
A low power laser is used in CD/DVD players as well as laser printers.
In a CD-ROM drive the laser will have 5mW whereas a DVD player will have a laser ranging from 5-10 mW. Burners need a higher powered laser and can range from 100-250mW, depending on the quality of the player and what kind of data is being burned.

Several difficulties have arised as this technology came about, however advancements in technology ahev been made to ensure that the CD is as efficient as possible. EFM as mentioned earlier is useful for preventing defects in a disc, however it is also useful in making sure there are not exteneded gaps where there lands occur, as this will set the tracker off track, so by using EFM 8-bit bytes are converted to 14 bits, preventing larger lands on a disc. Subcode data is the term used for the encoding of the absolute relative position of the laser on a track, this basic terms describes where a specific set of data is on a track. CDs GPS Lighthouses Global Positioning Systems are devices that were developed in 1973 with the intention of being used as a tool in defence. It was originally run with 24 satellites. This development overcame issues and limitations that were evident in earlier navigation systems that were used. It is now commercially available and useful in providing a location and current time to the receiver. GPS is a useful tool in areas of science, commerce, tracking and surveillance. Because it provides an accurate time it is also useful in banking and mobile phone operations.
The design and idea of the GPS is based on earlier radio navigation systems that were developed and used during World War II. The system uses radio waves that have a frequency which is approximately between 1227MHz-1575MHz. Inspiration for the device also arose from the first man made satellite that was launched- whilst monitoring radio transmissions a team of scientists discovered that the frequency of the signal being transmitted by the satellite was higher as the satellite approached, like-wise the frequency lowered as the satellite moved away from them. This occurrence was a demonstration of the Doppler effect. By measuring the Doppler distortion the scientists realised that they would be able to calculate the exact positioning of the satellite as they knew their own location on the earth. The first navigation system which proved successful in 1960 used five satellites and could provide a location around one time per hour. Further developments lead to a satellite which could give an accurate time as well as basic navigation systems.
GPS satellites transmit signals to sources located on the ground The receivers passively receive signals transmitted by the GPS satellites, they need un unobstructed view of the sky, meaning they will not work as well in forest areas or near tall buildings. Like radar systems, the receiver on a GPS calculates its exact positioning by timing the transmission of the signals that are sent out to the satellites. As little as four satellites are used for GPS to work for accuracy. The level of accuracy varies depending on the type of receiver used. When GPSs first came to use to non-military users the accuracy was limited to around 100 metres, this was known as selectively availability which was eliminated in 2000. Presently, a hand held GPS will have an accuracy of 10-20metres. Other receivers may use a method known as Differential DPS, this method is used to obtain a higher accuracy by using an additional stationary receiver that is in a nearby location. Positions are corrected by the stationary receiver and the accuracy can be as high as 1 metre.
The Earth's atmosphere, particularly the ionosphere can affect the speed at which a signal is being sent, causing inaccuracies and errors in readings. These effects can be quite small for overhead satellites but have a greater affect on satellite at the horizon, this is because the path through the atmosphere is longer at this point. However, when an approximate location of the receiver is known it is possible for the errors to be estimated and compensated for. As well as this humidity also causes the speed of a signal to be delayed and because humidity can change more quickly than the effects ionosphere has on a signal, it is much more complicated to obtain a precise measurement and compensate for humidity errors.
If a receiver is undergoing changes in altitude there can also be a change in the speed at which a signal is received as it is passing through less of the atmosphere when located at a higher altitude. GPSs are designed to produce an approximate altitude reading though and this error does normally not present greatly affected results in accuracy.
Overall there are many factors that contribute to errors and inaccuracies in readings however these are not extreme and with new advancements GPSs are being developed to eliminate these errors.
The lighthouse is one of the earliest communication technologies known. A lighthouse uses visible light which can be manipulated by mirrors and lenses to create a strong beam of light- used to guide ships at night or in stormy conditions when there is less sunlight. It is an extremely useful invention, preventing accidents and giving sailors ease when sailing in dark conditions. The Egyptians were the first to build a light house, using fire as a source of light. Later, wick lamps were used as a source of light however the beam could not travel as far as today due to lack of knowledge of how to manipulate light.
Future of Communication Technologies Lands and pits on a cd Visible light is a form of electromagnetic radiation. The human eye can view light waves that are about 400-750nm. the triumphs and the challenges Bluetooth is a technology developed by Ericsson in 1994, it uses short length radio waves to exchange information over short distances. It can be used with fixed or mobile devices and creates a personal area network. Bluetooth overcomes synchronization problems, allowing connections between several devices at once. It ranges from 2402-2480MHz and transmits data from 1Mbit per second to 3 Mbits per second. Bluetooth is a cheap and secure way of short distance transfer of data,and has become the standard, low power means of short distance transfer between devices.
Wireless internet is a recent technology that has benefited society in its ways of communication in a number of ways from communication by email to online news. It has also created a realtively inexpensive way for countries and regions with poor telecom infrastructure as well resources to be connected to the internet. Wireless Internet works on the same concept as the radio. To use wireless internet a wireless router is needed, this is a box that is plugged into a telephone socket and can link computers to for a local area network (LAN) as well as provide internet access. Instead of using cables to send and receive information, it is sent using radio waves. A router has a maximum range of around 90 metres, but there can be interference from other electronic equipment that can affect the signals. In order for a computer to receive infromation from a wireless router, it must have a radio transmitted in it or connected to it, these can be in the form of a wireless card which is built into a computer or connected via a port on the computer.
Although this has an enormous amount of capabilities there are also drawbacks to this technology. Wireless networks can be much more vulnerable when not properly protected from intruders or hackers. Wireless communication involves the transfer of information from one device to another, using waves instead of wires. Wireless technology has greatly enhanced the way the world communicates. This form of communication can be used over short or long distances.
In the late 1800s, physicist Heinrich Hertz made the first electromagnetic radio waves in his lab, around ten years later the first message was sent using radio waves. By 1899 it was possible to send radio waves across longer distances, such as the English channel. Later developments led to two-way transmitters and now more recently wireless internet. Wireless Networks Radios Radio waves have the longest wavelengths and the shortest frequencies, as well as having the lowest amount of energy on the electromagnetic spectrum. These waves can transmit information such as music, verbal conversations, pictures and data. Signals are transmitted by the modulation of electromagnetic waves, which have frequencies that are below visible light. Information travels by the means of oscillating electromagnetic fields which pass through the air and vacuums. The waves pass through an electrical conductor, usually an antenna in the case of radio and analogue television, when this happens the oscillating fields produce an alternating current in the conductor, which is then detected and transformed into sound or other information. This method of sending and communicating information has become useful in creating technologies such as Wireless internet and bluetooth.
Waves travel through space, their path can be direct or altered by reflection, refraction or diffraction. Their intensity decreases as a result of geometric dispersion and some energy may be lost by means of absorption through mediums that interrupt a waves path.
When radio first came about the transmitters were referred to as 'spark coils' as they created a continuous stream of sparks at a much higher voltage than is used to today, and a radio was referred to as a 'wireless' because it used no wires. This higher voltage in the original radios meant that the transmitters could transmit information farther. However, because there is such a wide range of frequencies a receiver could pick up transmissions and mix them up with other transmissions that are being sent. Radios in society today use continuous sine waves to transmit information, each different signal uses its own sine wave frequency, separating them all form each other. For example, a certain radio station is 106.8 on the dial, this means the station is 106.8 megahertz, the transmitter at the radiostation is oscillating at a frequency of 106,800,000 cycles a second. A station is continually broadcasting that signal at that certain frequency, when someone at the station speaks into a microphone or plays a record, they are changing the signal, when a radio picks up this signal the speakers on their radio vibrate the signal that has been changed and the listener will hear what was originally coming from the station. This signal that is sent is called the carrier frequency, because it carries the sound from the broadcasting station to a receiver such as a radio. FM radio stations are given their own band of frequencies, this band of the radio spectrum is speicifically used for FM radio and no other purpose. The case is the same for AM radios and television- they are all allocated their own band.

Two Way Radios Television Radar Cordless Phone Mobile Phones Two-way radios were one of the first devices that allowed long-distance communication, they are still commonly used in industries today. Hertz's experiments in 1885 with electricity and electromagnetic waves gave the information needed to develop the two-way radio. The two-way radio replaced carrier pigeons and flags on ships. This made communication easy and safe, however there were still limitations, such as the inconveniently large size of these devices.
The television works in a similar way to the radio in that, information travels along a carrier frequency from a broadcasting station and is picked up by a receiver on the television which is normally an antenna or satellite, and this information is then converted back into it's original form of audio and visual and displayed on the screen of the television. The basis of the television existed as early as the 1830s however it wasn't until the 1940s that the television became a practical invention that was mass-produced for the public.
With the development of radio, x-rays and the advancements in physics in the early 1900s this idea of transmitting an image to a receiver could be made into something available for mass production. Karl Braun discovered that a cathrode-ray tube could be manipulated by an electron stream with a magnetic field and with this information, in 1906 it was suggested that the cathrode-ray tube could be used as a device for receiving images and soon a cathrode-ray tube was developed that could do this.
Shortly after World War I an 'iconoscope' was created. This device was used to convert an image into electronic signals, ready for transmission. The data being sent was minimized by feeding the image through a plate covered with microscopic dots, or what is known as pixels today. This image could be sent and received using a cathode-ray tube. This became the first real television and was demonstrated to the public in 1929. However the television was still being modified to become more practical for the homes. The concept of radar is quite similar to radio technology, and like a radio or television, radar uses a transmitter and receiver, sending out pulses in short bursts of electromagnetic radiation which is then picked up by the receiver. By timing how quickly the receiver is picking up these pulses and the strength of the returning signals radar technology can be used to determine the distance a specific target is from the transmitter.
The first radars used an oscilloscope to convey how far an object was, timing the returned signal and displaying it as a video signal- showing up as a peak on the display. Radar has become useful in aviation and all aircraft contain radars which are used to detect and warn the pilot of any obstacles that may be approaching their path of flight, as well as giving altitude readings. Radars are also used in this same way on ships and other marine vessels.
There are several factors which limit the accuracy of radars; noise and interference often occur and radar systems need to overcome any unwanted signals to focus on a specific target.
Signal to noise ratio (SNR) is a radar system's ability to overcome any interferences, the higher a radar systems SNR is the better it will be in avoiding any unwanted interference or noise.
Clutter is also a factor which can affect the accuracy of a radar system's detection. Clutter is the radio frequency echoes which return from the targets that are unnecessary to the users of a radar system. This may be echoes from the ocean, ground, rain animals or buildings. Different ways of polarization are used to remove clutter in radar systems. In most cases a radar will be locating a moving target so any stationary targets that are picked up on the system can be eliminated as they are usually clutter.
Due to a variation of the refractive index of air a radar beam will not follow a linear path when traveling through the atmosphere, the path is to some extent curved. Because of this tendency there is a maximum height above ground that a conventional radar can correctly receive information from. Cordless phones use the same principles as a radio. They using high frequency radio waves and can transmit as well as receive signals which are then converted into sound and made audible to the user. The cordless phone became a useful invention as it meant that the user would not be restricted by short distances of a cord when using a telephone.
There is a base unit that is connected to the main part of the phone whilst the handheld phone has a small battery in it, which can be charged- although the phone is cordless, the base unit of this device still needs to be plugged in to charge.
The first cordless phone was introduced in the 1970s, and then later in 1986 a greater frequency range was allowed for cordless phones which meant that problems to do with interference were reduced, later this frequency range increased once again, this time to a range of 900MHZ. A few years later the digital cordless phone was introduced which meant that the phones now worked on the digital spread spectrum (DSS) this was intended to reduce the amount of unwanted eavesdropping in phone calls, and ensure more security in the privacy of phone calls.

The first mobile telephony service was in Sweden in the mid twentieth century. This service was a form of radio telephony- a two-way radio which is still used in taxis and by police today. This became the basis of the mobile phone technology used today. In 1946 the first commercial mobile phone service was bought out- this service was a means of communication which was permanently installed in vehicles and used a weaker signal and much lower bandwidth than mobile phones currently use today. Then the first mobile phone system was brought out by a company known as Motorola in the 1970s and the popularity of this technology increased greatly in the 1990s and by 2003 there were about 1.52 billion mobile phone users around the world. Today new models of phones are constantly being created and modified to provide not just a means of calling another phone user and sending text messages but a camera, audio player, voice recorder and portable internet browser.
Although there is argument that mobile phones can cause damage to people- in particular radiation from the device causing brain tumors and disturbed brain function. However little research has been done to confirm whether they are harmful.
AM radio has been around since the early 1900s, its capabilities were limited due to its low frequency allocation. AM stands for Amplitude Modulation, and works by varying the strength of a signal that is transmitted. The strength of the signal depends on what data is being sent. This contrasts with FM, which stands for Frequency Modulation, where the frequency is varied depending on the information being sent. With the invention of FM radio higher-qualtiy transmission was available to the public. Two-way radios were used in police vehicles and initially they took up the entire back seat, although with further research and developments two-way radios could be hand-held or permanently installed into cars and could work over greater distances.
Basically, a two-way radio is a radio with a microphone and a speaker and unlike the mobile phones we use today, which can simultaneously send and received information, the first two-way radios could only do one job at a time. To successfully use one both the sender and receiver must have their radios on the same frequency in order to pick up the signal. Today two-way radios use either Ultra High Requency (UHF) or Very High Frequency (VHF). And the signal is sent through a vacuum by radio waves, much the same as the way sound is transmitted from a broadcasting radio station to a radio, however in this case from one radio to another. Paul Nipkow, a German engineer created a device in 1833 that became the basis of the idea of the television. his device was a rotation disk that had small holes in a spiral pattern on it. A sensitize photocell was placed behind the rotating disk and an image could be broken down and transmitted by the photocell in a series of electrical impulses which would be sent to a receiver. The receiver would convert this electricity into light and shine it through another spinning disk, which was identical to the original disk. The result was a reconstruction of the original image, however it was of poor resolution. This invention gave inspiration to other electronics enthusiasts who made imitations and variations of Nipkow's invention. However these inventions were impractical for commercial use. Bluetooth Mobile phones play a big role in society today in means of communication, they allow phone calls to be made without the limitations of regions and time. Once again, using the same concept as radios mobile phones receive signals but also give out their own signals which are received and transmitted by towers and satellites and then transmitted and received by another mobile or landline phone. Mobile phones use a higher frequency than radios, providing clearer reception. A man by the name of Augustin Fresnel found a way of increasing the intensity of the light by using prisms and in 1841 his lens which he named the Fresnel lens was installed in a lighthouse for the first time. Lighthouses required regular maintenance as they were constantly under harsh weather conditions such as rain, wind and salt-spray. Originally they were run by a keeper, which meant the keeper would be kept extremely busy maintaining the lighthouse and working shifts to keep the light running, however today most lighthouses use electricity so a keeper is not necessary. Timeline 280 BC Approximation of first lighthouse built. 1822 Development of Fresnel lens, making manipulation of light easier, specifically for lighthouses. 1830s Nipkow develops an invention that became the basis of the idea of television 1890 Two way radios invented 1900s First broadcast of entertainment radio as well as the first audio transmission by radio 1930s Radar technology is developed 1940s Invention of the television, available now the the public. 1970s Cordless phone is introduced as well as Motorola producing mobile phones. 1973 GPS is developed and used in the military. 1980s CD technology invented. 1994 Bluetooth is developed 1997 WiFi technologies are developed Today electronic devices for communication are constantly being developed and research is being done to make these devices smaller, faster and sleeker looking than the last model. Here are some examples of technologies that are being developed for the future...
Currently a phone that is foldable, much like a swiss army knife, is being developed. It can be used as a gaming platform and a multimedia player. It's high resolution screen is flexible enough to be able to fold in half. However these communication devices are not just being developed to be smaller, there is also device that is being developed by a group of scientists at the VVT Technical Research Center in Finland which would be used as a security device installed on a phone or laptop. The device has a sensor that has the ability to analyze as well as save the walking patterns of the owners, and if the device detects an unfamiliar walking pattern (ie. is not being carried by it's owner) the device will trigger the laptop or mobile phone into a lockdown mode where a password will need to be entered to unlock it, making the phone or laptop useless to anyone else. This technology will ideally give thieves no point in stealing these communication devices if this sensor device were to become commonly used amongst phone and laptop users. Other developments are being done in areas in Europe in car manufacturing. Soon all of the cars being manufactured will have devices installed in them can receive warnings from sensors located on the roads to do with road conditions as well as broadcasting warning messages. With this kind of technology more traffic accidents would be able to be avoided. There are also developments being made in creating virtual keyboards which are pocket-sized, portable and futuristic looking. They use an infra-red light to display the keyboard in front of the device and work using detection. Currently they are available to the public, but newer better working versions are being designed to further assist computing and using portable devices.
However one of the main focuses of research is on creating a device that would be able to do everything electronic you need, so ideally you would just have one portable device, rather than multiple mobile and stationary devices. This diagram gives an idea of where these technologies are in relation to the electromagnetic spectrum...
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