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Transcript
1st November 2008 Name: Tomasz Waśniewski Course: Computer Network Administration Report: Electromagnetic waves Page 1 Introduction wall like they make Electricity can be static, like what holds a balloon to the or makes your hair stand on end. Magnetism can also be static a refrigerator magnet. But when change or move together, they waves – electromagnetic waves. Electromagnetic waves are formed when an electric field couples with a magnetic field. The magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave. James Clerk Maxwell and Heinrich Hertz are two scientists who studied how electromagnetic waves are formed and how fast they travel. About 150 years ago, James Clerk Maxwell, an English scientist, developed a scientific theory to explain electromagnetic waves. He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. Neither an electrical field (like the static which forms when you rub your feet on a carpet), nor a magnetic field (like the one that holds a magnet onto your refrigerator) will go anywhere by themselves. But, Maxwell discovered that a changing magnetic field will induce a changing electric field and vice-versa. An electromagnetic wave exists when the changing magnetic field causes a changing electric field, which then causes another changing magnetic field, and so on forever. Unlike a static field, a wave cannot exist unless it is moving. Once created, an electromagnetic wave will continue on forever unless it is absorbed by matter. Heinrich Hertz, a German physicist, applied Maxwell's theories to the production and reception of radio waves. The unit of frequency of a radio wave - one cycle per second - is named the hertz, in honour of Heinrich Hertz. Hertz proved the existence of radio waves in the late 1880s. He used two rods to serve as a receiver and a spark gap as the receiving antennae. Where the waves were picked up, a spark would jump. Hertz showed in his experiments that these signals possessed all of the properties of electromagnetic waves. With this oscillator, Page 2 Hertz solved two problems. First, timing Maxwell's waves. He had demonstrated, in the concrete, what Maxwell had only theorized that the velocity of radio waves was equal to the velocity of light! (This proved that radio waves were a form of light!) Second, Hertz found out how to make the electric and magnetic fields detach themselves from wires and go free as Maxwell's waves. Page 3 Laboratory equipment In the laboratory, during our experiments, we used the following equipment: - Horn Transmitter (Tx) and Receiver (Rx) [to originate and receive the signal] Two dual-height stands [to support both Tx and Rx] Micro amp Meter [to get the readings from Rx] Power Supply Unit [to supply Tx with electric power] Copper cables [ to connect Rx to Micro amp Meter and Tx to Power Supply Unit] The length of the wave that Transmitter produced was 2.8 cm. Wanting to know its frequency (f) we calculated it using the following equation: f = c / λ , where c= 3*108 m/s [speed of light] and λ = 2.8 cm = 0.028 m [wave length] Putting the numbers in the place of letters we came out with the actual value of frequency that both devices (Tx and Rx) were working on: f = (3*108 m/s) / (0.028m) = 10.714 GHz Voltage that we used for our Transmitter was 12V. We could sketch a graph showing a sine calculating earlier the periodic time (T): T = 1 / f = 1 / (10.714*109) = 0.09 ns Page 4 wave we were using, Experiment 1 – Signal strength and polarisation During the first experiment we analysed the basic properties of electromagnetic waves and measured the strength of the signal depending on different conditions. We placed the Transmitter and Receiver opposite to each other as shown on the diagram below: We noticed that as the distance between the two devices was getting bigger the less current got to the receiver. Following the same logic as we brought them closer to each other the signal was stronger. We also tried to put different materials between the Tx and Rx and got different readings written down (the distance between transmitter and receiver was equal to 1 meter): - paper (12 µA) glass (11 µA) cardboard (6 µA) copper plate (0 µA) My personal notice was that the thicker and denser the material the poorer the signal was. And so paper almost did not affect the signal at all. Glass had a minor impact, while thicker materials like copper plate or aluminium nearly did not let the electromagnetic waves through. Later on we tried to put those materials close to, but not on the direct path of the waves. The results varied, but there was one interesting thing worth to mention here. When we placed the metal grid parallel to the wave path, close to the transmitter, the strength of the signal received by receiver was much stronger. This is probably more because of reflection though. Polarisation Polarisation is a property of waves that describes the orientation of their oscillations. For transverse waves, it describes the orientation of the oscillations in the plane perpendicular to the wave's direction of travel. For electromagnetic waves such as light, the polarisation is described by specifying the direction of the wave's electric field. Page 5 Polarisation has many applications among different domains of science. It is widely used in biology, geology, chemistry, astronomy and even in art. One of the most common inventions using the laws of polarisation is probably sunglasses. These work as a polarisation filter, and enable our eyes to look straight into sun. Polarising filters remove light polarised at 90o to the filter's polarisation axis, and so they can be used to observe the effect of palarisation by looking through them at the reflected light at different angles. Another interesting use of polarisation has been observed in nature. Many animals are capable of perceiving the polarisation of light, which is generally used for navigational purposes, since the linear polarisation of sky light is always perpendicular to the direction of the sun. This ability is very common among the insects (like bees) or other species like octopus, squid, cuttlefish or mantis shrimp. The naked human eye is weakly sensitive to polarisation, without the need for intervening filters. Polarised light creates a very faint pattern near the center of the visual field, called Haidinger's brush. This pattern is very difficult to see, but with practice one can learn to detect polarised light with the naked eye. All radio transmitting and receiving antennas are intrinsically polarised, special use of which is made in radar. Most antennas radiate either horizontal, vertical, or circular polarisation although elliptical polarisation also exists. Vertical polarisation is most often used when it is desired to radiate a radio signal in all directions such as widely distributed mobile units. AM and FM radio uses vertical polarisation. Television uses horizontal polarisation. Alternating vertical and horizontal polarisation is used on satellite communications (including television satellites), to allow the satellite to carry two separate transmissions on a given frequency, thus doubling the number of customers a single satellite can serve. Polarising filters are also used in photography. They can deepen the color of a blue sky and eliminate reflections from windows and standing water. Polarisation is used also in art. Some artists apply special techniques using polarisation rules to create colorful and often changing images. 3-D movies are based on polarisation too, but the 3-D effect works only on a silver screen since it maintains polarisation, whereas the scattering in a normal projection screen would void the effect. The last example of a device using polarisation laws I will mention about is Liquid Crystal Display (LCD) screen. Nowadays it is replacing old Cathode Ray Tube (CRT) monitors and television sets, as the LCD monitor provides better quality Page 6 of image and takes much less space. It also uses small amount of power and so it is very commonly used in small electronic devices such as calculators or alarm clocks. To prove that the devices we used during these labs were vertically poralised we used metal grille rods and put them between transmitter and receiver. First we put them vertically. No signal was received by the receiver. Then we changed the position of the rods and put them horizontally. This time signal was not affected by them and we got the same reading as if there was nothing between the Tx and Rx. That experiment proved that our waves were vertically polarised, because when we put metal grille rods vertically on the wave path they were picked up by the rods which acted in this case as the antenna. Finally we did one last experiment. We turned the receiver for and measured the signal. The reading was 0, what was a proof that the waves we were using were vertically polarised. 90o Page 7 Experiment 2 – Reflection Our second experiment was about the basic reflection of electromagnetic waves. Again we measured the strength of the signal – this time depending on the position of transmitter and receiver and the material reflecting the signal and the amount of reflections. General setting looked very much like this below: During this experiment we tried to keep both the transmitter and receiver at the same angle of 45o to the reflecting material as we know from the lectures that the angle of incidence is equal to angle of reflection, and so to get best readings it is best to keep the wave path as close to the Receiver as possible. The initial distance between two devices pointed at the reflecting material at an angle of 45o was 1 m. As both angles of incidence and reflection were making 90o angle, and we knew that the distance between transmitter and reflecting material was approximately equal to the distance between receiver and reflecting material, we could work out those approximate distances from the Pythagoras’ theorem: (1 m)2 = a2 + a2 = 2a2 or Rx and reflecting material , where a is a distance between Tx And so we came out with the value of a: a = 12 / 2 = 0.707 m , which is approximately 70 cm. Basic reflection First we tried different reflecting materials. We used very similar materials to those we used during first experiment. Similarly we got different reading, only this time the results were opposite: - paper (0 µA) glass (0,5 µA) Page 8 - cardboard (2 µA) copper plate (8 µA) aluminium plate (15-20 µA) From this instance I came to conclusion that the more dense and firm the material was the better it reflected the wave. The interesting notice is that the aluminum plate, though lighter than copper, reflected the signal very effectively. This is probably because aluminum is very similar to silver when it comes to reflecting transverse waves, as the silver is a very good reflecting material. The other materials voided the signal more or less, reflecting only part of it. Next we tried to move the devices, putting them at different angles and closer or further from the reflecting material. Results were similar to the previous experiment – the further the distance the weaker the signal. What about slant, just as I stated before, keeping devices at the approximately same angles made more current received by the receiver. Advanced reflection After experimenting with basic reflection rules, our curiosity brought us to create more complex setting and include more than one reflecting material: We tried to bounce the signal multiple times before it reached the Receiver and our results varied a lot. Depending on the path we built we got stronger or weaker signal. In some cases we could not get signal at all. Generally the more reflectors we used the weaker the strength of the signal was. Some particular settings were making the signal stronger though; these were settings in which reflecting materials bound the signal inside the area between transmitter and receiver letting only small parts of it escape. - Optical fiber An optical fiber became an alternative to copper cable and wireless networks. It is a glass or plastic wire that carries light instead of current. Page 9 Optical fiber is able to carry much more data (it has a wider bandwidth) than any other kind of media (wireless or copper cable for instance). It is also much faster and more efficient than other forms of communication, as well as it allows transmission over longer distances. Fibers are immune to data loss, noise and electromagnetic interference. Optical fibers use a specific rule of reflection called ‘total internal reflection’, which means that the light wave is bound within the core of the fiber, which acts as a waveguide for the light it carries. It is built from two different kinds of glass, having two different refraction indexes (refraction index of cladding layer must be lower than the refraction index of core material). The larger the index of refraction, the more slowly light travels in that medium. The output is that the light wave is reflecting from the ‘cladding’ layer glass and thus moving further through the fiber. The angle at which the light wave is inserted is critical, as not everyone will work and make the wave reflect. See the diagram and image below: There are two general modes optical fibers work on: - Single Mode Fibers (SMF) – working only on one mode; used for links longer than 200 meters Multi-Mode Fibers (MMF) – supporting several modes at the same time; usually used for short-distance communication Optical fiber has many applications, most of which are connected with networks and communication. It is definitely a revolution among media world, and it will probably take place of copper cables in the nearest future. Optical fiber is able to deliver information over very long distances with repeaters only every 50-80 km. Erbium-doped fiber allows the signal to regenerate without using any electrical power, which means it is far more efficient and effective. Another big advantage of optical fiber is its bandwidth, capable of caring more data at the same time, measured not in Mb/s but Gb/s! The latest rumors say that speeds of 1Tb/s are being developed! Fiber is also immune to electrical interference, which prevents cross-talk between signals in different cables and pickup of environmental noise. Other uses of optical fiber include medicine, illumination applications (like this Frisbee illumination on the picture Page 10 beside), decorations and art. In some buildings, optical fibers are used to route sunlight from the roof to other parts of the building. A lot of signs or images use optical fiber for illumination effects either to make it more visible or just because of its original effect, or often both. Fibers have many uses in remote sensing. In some applications, the sensor is itself an optical fiber. In other cases, fiber is used to connect a non-fiber optic sensor to a measurement system. Depending on the application, fiber may be used because of its small size, or the fact that no electrical power is needed at the remote location, or because many sensors can be multiplexed along the length of a fiber by using different wavelengths of light for each sensor, or by sensing the time delay as light passes along the fiber through each sensor. The last experiment on reflection we did was a very specific setting of transmitter and receiver put beside each other. Opposite them we put a reflecting material, so that overall setup looked exactly like that one below: Again we measured strength of the signal depending on different materials and came out with very similar results and conclusions as with the basic reflection instance. That settings reminded us about a specific device, called radar. - Radar Radar is an acronym for Radio Detection and Ranging. The term became used as a new word and entered English language with time, and is no longer written using capital letters. Radar is electromagnetic range, altitude, both moving and Page 11 a device that uses waves to identify the direction, or speed of fixed objects such as aircraft, ships, motor vehicles, weather formations or terrain. It transmits the signal, which is being reflected by the target and pick up the reflected signal by the receiver, which usually located very close to the transmitter (as on the diagram above). The signal that returns is often quite weak, but this does not make a big problem as the signal can be amplified. This makes radar a very useful tool, even at long distances. Radar devices use different frequencies, ranging from 3 MHz to 300 GHz, depending on the particular function the device is used to. For example, air traffic control stations use radars that work on 12 GHz frequencies. Radar is used in many contexts, including meteorological detection, measuring ocean surface waves, air traffic control, police detection of speeding traffic, and by the military. Further explorations Below there are a few additional topics and examples concerning electromagnetic waves. - Electromagnetic spectrum Waves in the electromagnetic spectrum vary in size from very long radio waves the size of buildings, to very short gamma-rays smaller than the size of the nucleus of an atom. The electromagnetic spectrum should be familiar to most of us. The microwave we use to heat our food and the cell phones all use waves that are part of the Electromagnetic Spectrum. The light that our eyes can see is also part of it. This visible part of the electromagnetic spectrum consists of the colours that we see - from reds and oranges, through blues and purples. Each of these colours Page 12 actually corresponds to a different wavelength of light. - Modulation Modulation is the process of varying a periodic waveform, like a tone, in order to use that signal to convey, in a similar fashion as a musician may modulate the tone from a musical instrument by varying its volume, timing and pitch. Usually a high-frequency sinusoid waveform is used to carry the signal. The three key parameters of a sine wave are its amplitude ("volume"), its phase ("timing") and its frequency ("pitch"), all of which can be modified in accordance with a low frequency information signal to obtain the modulated signal. Modulation is used to make the signal stronger and carry it over long distances, as well as regenerate it. A device that performs modulation is called a modulator, and one that does the inverse task is called a demodulator. Very often a device that can do both jobs is used and it has a well known name of a modem (modulator-demodulator). - Maxwell’s equations Maxwell's equations are a set of four partial differential equations that describe the properties of the electric and magnetic fields and relate them to their sources, charge density and current density. These equations prove that light is an electromagnetic wave. Individually, the equations are known as Gauss' law, Gauss' law for magnetism, Faraday's law of induction, and Ampere’s law with Maxwell's correction: xH= 0 E/ t+j xE=- 0 H/ t . H=0 . E= / 0 , where H is the magnetic field (A/m), E is the electric field -12 (V/m), j is the vector current density (A/m2), 0 = 8.8542 x 10 -7 F/m is the permittivity of free space, x 10 H/m is the 0 = 4 permeability (C/m3). of free space, and is the scalar charge density Gauss' law describes how electric charge can create and alter electric fields. In particular, electric fields tend to point away from positive charges, and towards negative charges. Gauss' law is the primary explanation of why opposite charges attract, and like repel: The charges create certain electric fields, which other charges then respond to via an electric force. Gauss' law for magnetism states that magnetism is unlike electricity in that there are not distinct "north pole" and "south Page 13 pole" particles (such particles, which exist in theory only, would be called magnetic monopoles) that attract and repel the way positive and negative charges do. Instead, north poles and south poles necessarily come as pairs (magnetic dipoles). In particular, unlike the electric field which tends to point away from positive charges and towards negative charges, magnetic field lines always come in loops, for example pointing away from the north pole outside of a bar magnet but towards it inside the magnet. Faraday's law of induction describes how a changing magnetic field can create an electric field. This is, for example, the operating principle behind many electric generators: Mechanical force (such as the force of water falling through a hydroelectric dam) spins a huge magnet, and the changing magnetic field creates an electric field which drives electricity through the power grid. Ampere’s law with Maxwell's correction states that magnetic fields can be generated in two ways: By electrical current (this was the original "Ampere’s law") and by changing electric fields (this was Maxwell's correction, also called the displacement current term). Page 14 References • http://science.hq.nasa.gov • http://en.wikipedia.org • Physics – Hans C. Ohanian • AIT Telecommunication Fundamentals – Dr. Robert Stewart Page 15