Electromagnetic Radiation and Human Health
Electromagnetic waves cover a wide range of frequencies or wavelengths, as shown in Figure 1.1. The classification is based mainly on the sources of Figure 1.1 Electromagnetic spectrum. The interaction of electromagnetic waves with matter depends on the energy of photons. In general, shorter waves corresponding to energetic photons interact more strongly than longer waves. The photons of radio waves have low energies for example, at 1,000 GHz the energy is only 4 X 10 3 eV (1 eV 1.6 X 10 19 Ws 1.6 X 10 19 J). The energy needed to ionize molecules in biological tissue is at least 12 eV. Thus, ultraviolet Human beings gather a lot of information through electromagnetic waves. The retina of our eyes is sensitive to visible light, that is, wavelengths from 380 nm to 780 nm. The human skin can sense infrared or thermal radiation. Other parts of the spectrum cannot be sensed directly they require their own specialized techniques to make the information carried by electromagnetic waves...
3.1 Basic Properties of Electromagnetic Waves The propagation of an electromagnetic wave is described through different parameters the electric and magnetic fields E and H, the electric flux density D and the magnetic induction B (Jouguet 1978). Only vectors E and B generate effects allowing the determination of the electromagnetic field. The vectors D and B are linked to vectors E and H through the following linear relations
The road to the practical application of frequency division multiplexing opened up when it became technically possible to (i) generate electromagnetic waves with well specified frequencies, (ii) modulate them by voice, (iii) amplify them without distortion and (iv) separate the different frequencies at the receiver. All these technical problems were solved by the middle of the 1930s. In 1938 the London-Birmingham trunk line was opened to service. It could carry as many as 40 simultaneous telephone conversations.
The first geostationary satellite, Early Bird, was launched in 1965. It provided one television channel or 240 two-way telephone channels not a very large number but, at the time, it effectively doubled the transAtlantic telephone capacity. It has to be noted here that geostationary satellites are not ideal for telephone conversations. The reason is the finite speed of electromagnetic waves carrying the message. The geostationary orbit is 36000 km above the Earth hence the waves carrying
Of greatest relevance to the advance of communications was the work on radar. After all, what radar does is to radiate in a narrow direction a pulse of electromagnetic waves which is reflected by the target and, from the time taken for the pulse to come back, the distance of the target from the radar set may be determined. The emphasis was on electromagnetic waves. The same waves that had been used for communications since the end of the last century. Hence the advances made during the war in producing electromagnetic waves could be easily translated to use those same waves for communications. The frequency range used by radar was in the microwave region (see Fig. 6.2 for the electromagnetic spectrum) hence microwave links for communications were a simple continuation of the work done during the war. A second advance was also due to radar. The devices which detected the reflected electromagnetic waves were made of semiconducting crystals. A semiconductor is neither a good nor a bad...
If the aim is to guide the light along the dielectric waveguide then it is strictly necessary for the index of refraction inside to exceed that outside. Since the guide cannot levitate in air and is in need of some support, this condition will not apply at the points of support where the electromagnetic energy may leak out. The way to overcome the problem is to clad the dielectric waveguide in a suitable material. It turns out to be advantageous to choose a cladding material whose index of refraction is just slightly smaller than that inside. Such an arrangement would allow single mode operation with a reasonably sized, not too small, inner core. Single mode operation is preferable because, as mentioned for hollow metal waveguides, it can avoid distortion of the signal.
Hertz's apparatus was shown in Fig. 6.3. It worked. It was the first one that worked nonetheless it is not the best for explaining the underlying principles. From a more modern viewpoint, an apparatus that will radiate electromagnetic waves may be represented schematically by the block diagram of Fig. 6.4.
Out of the blue. And still they turned out to be correct. It was a great event in the history of science but, as it turned out, it was also a great event in the history of engineering. True, it took a generation from Maxwell postulating his equations to Marconi making the first use of electromagnetic waves, but eventually the engineers got there. Signals could be picked out of thin air.
For Reeves the advent of the laser came as an unexpected boon. His men were already running along the right track when a bandwagon suddenly appeared. They jumped upon it without losing any time while the rest of the world was not even aware that there were any bandwagons on the move. Reeves and his co-workers looked at various guiding structures the hollow metal tube already mentioned an array of lenses (called confocal waveguides because the light in them was focussed and defocussed as it propagated from lens to lens, see Fig. 12.2) and the dielectric waveguide which had already existed in the form of bundles of glass fibre and which had been used for medical purposes. A dielectric is nothing more than an insulator, a material that does not conduct electricity. The mechanism by which they could guide electromagnetic waves had been known for a century at least. For a qualitative explanation see Box 12.1.
Waveguides are made of hollow metal tubes which may have rectangular or circular cross-sections, as shown in Fig. 9.6. The electromagnetic waves carry the information inside the tubes very much like water. Information flows in at one end and comes out at the other end. An alternative view is to look at wave propagation as a result of a series of reflections by the walls, as shown in Fig. 9.7 for a rectangular waveguide.
Arrived in the company of his mother who had good connections in London society. He wrote first to the War Office but when the reply was long in coming he succeeded, thanks to his mother's contacts, in obtaining an introduction to William Preece, the chief Engineer of the Post Office. We met him in the last chapter as the adversary of Lodge, the villain unwilling to believe in the beneficial effects of self-induction. Concerning wireless, was he a hero or a villain He could have been a great hero by supporting the unconventional ideas on light and electromagnetic waves just taking shape in England that came from the disciples of Maxwell. It would have perhaps been too much to expect him actively to help his professional enemies. That would have required selflessness and magnanimity few chief engineers are endowed with. It would however not be improper to regard him as a minor hero who was at least able to recognize a promising device when he saw one, and was imaginative enough to...
Of all the electromagnetic spectrum the part most studied in the course of human history is light. The main reason has been the early availability of powerful light sources ('Let there be light', Day 1) supplemented five days later by a marvellously effective broadband light
And then out of the blue came a theoretical proposal to reach frequencies a couple of thousand times higher. Two American physicists, Arthur Schawlow and Charles Townes, claimed that it would be possible in principle to produce electromagnetic waves which have frequencies of about 500THz.2 The corresponding wavelength is o.6 J,m Perhaps the simplest (although not quite accurate) way to define coherent waves is to say that the radiation is of a single frequency. The waves coming from the Sun contain a wide range of frequencies, as witnessed by rainbows. On the other hand the electromagnetic waves which bring us radio and television are close to a single frequency. For incoherent waves the 15000 channels mentioned above would have been a marvellous achievement. For coherent waves, for which our old calculations apply, it's just peanuts. Taking a 15 per cent bandwidth around 500 THz the number of telephone channels comes to about 18 billion with single sideband modulation, and still...
Two different experiments were thus conducted at France Telecom R&D in order to study the influence of rain on the propagation of radio waves (Gloasguen 1993 Veyrunes 2000). The first experiment was intended at measuring the bistatic scattering of electromagnetic waves by rain particles along a 312 metre path at the 94 GHz frequency in vertical polarisation (Gloasguen 1993). This experiment was conducted using two transmitting and receiving rotating antennas, which could both be rotated independently in the horizontal plane, thereby allowing the meas The second experiment was intended at simultaneously measuring the meteorological conditions and the variations of the radio field at the 30, 50, 60 and 94 GHz frequencies along an 800 metre path in line-of-sight. The statistical study of the resulting experimental data, concerning for instance the interactions occurring between electromagnetic waves and different phenomena of atmospheric and meteorological nature, or the development of...
The electromagnetic field inside buildings is subject to the influence of different parameters, including the position of the building with respect to the emitter and to other buildings or the architectural characteristics of the building, for instance the materials, the interior layout or the size of the windows. Values obtained for a given building should not, therefore, be generalised to any kind of building when performing coverage predictions. While a database integrating a large enough number of characteristics would help improving the precision of building penetration loss models, the development on a large scale of such a database remains difficult (Toledo 1998). - the nature of the materials. The attenuation that electromagnetic waves undergo varies from 4 dB in the case of wood to 10 dB in the case of concrete walls (COST231 1999).
Angular refraction through the atmosphere occurs because radio waves travel with differing velocities in different parts of a medium of varying dielectric constant. In free space the group velocity is maximum, but in the nonionized atmosphere, where the dielectric constant is slightly greater due to the presence of gas and water molecules, the radio wave travels more slowly. In what radiometeorologists have defined as a standard atmosphere, the pressure, temperature, and water vapor content (humidity) all decrease with increasing altitude. The dielectric constant, being a single parameter combining the resultant effect of these three meteorological properties, also decreases with altitude (Refs. 7-9). Since electromagnetic waves travel faster in a medium of lower dielectric constant, the upper part of a wavefront tends to travel with a greater velocity than the lower part, causing a downward deflection of the beam. In a horizontally homogeneous atmosphere where the vertical change of...
In some propagation models, the vegetation in urban areas is regarded as a shadowing region which prevents the propagation of electromagnetic waves (Cor-reia 1996). At the 60 GHz frequency, the authors indicate values ranging from 6 to 8 dB for the attenuation due to vegetation (Correia et al. 1994).
Tropospheric scatter diffraction sites will be larger, often requiring greater site improvement including fresh water, sanitary systems, living quarters, and more prime power and larger backup power plants. Radiated electromagnetic interference (EMI) is of greater concern. Takeoff angle (9et, der) is critical. For each degree reduction of takeoff angle there is a 12-dB reduction (approximately) in median long-term transmission loss.
Telecommunications technologies using electromagnetic transmission surround us television images flicker, radios chatter, cell phones (and telephones) ring, allowing us to see and hear each other anywhere on the planet. Email and the Internet link us via our computers, and a large variety of common devices such as CDs, DVDs, and hard disks augment the traditional pencil and paper storage and transmittal of information. People have always wished to communicate over long distances to speak with someone in another country, to watch a distant sporting event, to listen to music performed in another place or another time, to send and receive data remotely using a personal computer. In order to implement these desires, a signal (a sound wave, a signal from a TV camera, or a sequence of computer bits) needs to be encoded, stored, transmitted, received, and decoded. Why Consider the problem of voice or music transmission. Sending sound directly is futile because sound waves dissipate very...
In this work, we shall limit ourselves to the study of radio waves this term apply to the electromagnetic waves used in radio communications. Their frequency spectrum is very broad, and is divided into the following frequency bands ELF waves (f 3 kHz), VLF (3-30 kHz), LF waves (30-300 kHz), MF waves (300-3000 kHz), HF (3-30 MHz), VHF waves (30-300 MHz), UHF waves (300-3000 MHz), SHF waves (3-30 GHz), EHF waves (30-300 GHz) and sub-EHF waves (300-3000 GHz). Chapter 3 is entirely devoted to electromagnetic waves. These waves are the propagation mode of electromagnetic disturbances characterised by a simultaneous variation of an electric field and a magnetic field. Electromagnetic The first section of this chapter is devoted to the fundamental properties of electromagnetic waves and approaches the following topics the electromagnetic parameters, the electric and magnetic fields, the electric and magnetic induction, Maxwell's equations, the propagation velocity of a wave, the wavelength,...
A simple scheme, often employed in practice, controls the state of polarization (SOP) with which each channel is launched into the fiber link 52 . More specifically, individual channels are launched such that any two neighboring channels are orthogonally polarized. In practice, even- and odd-numbered channels are grouped together and their SOPs are made orthogonal before launching them into the fiber link. This scheme is sometimes referred to as the polarization channel interleaving technique. The XPM interaction between two orthogonally polarized does not vanish but its strength is reduced significantly. Mathematically, the analysis is complicated because one must take into account the vector nature of the electromagnetic field within the fiber 4 , It turns out that the coupled NLS equations, Eqs. (4.2.2) and (4.2.3), can still be used, provided the factor of 2 appearing in the XPM term is replaced with 2 3. It is this reduction in the XPM strength that reduces the magnitude of the...
The Scottish physicist and mathematician James Clerk Maxwell (1831-1879) predicted the existence of electromagnetic waves. He combined Gauss' law for electric and magnetic fields, Ampere's law for magnetic fields, and the Faraday-Henry law of electromagnetic induction, and added displacement current to Ampere's law. He formulated a set of equations, which he published in 1864. These equations showed the interrelation of electric and magnetic fields. Maxwell proposed that visible light is formed of electromagnetic vibrations and that electromagnetic waves of other wavelengths propagating with the speed of light were possible.
A transmission line is a system of conductors which connects two points by means of electromagnetic energy which travels along the system. A system may include wires which carry 240 V 50 Hz mains supplies to consumers, or it may consist of lines which feed thousands of watts to a transmitting antenna. There are various kinds of transmission lines in use - the twisted pair, twin lead and coaxial, to mention a few. Twin-wire lines are often used for carrying radio frequency power to transmitters, while coaxial cables are commonly employed for low-power applications such as coupling receivers to their antennas.
It is instructive at this point to explain what is meant by a standing wave. A wave travelling along a transmission line consists of electric and magnetic components, and energy is stored in the magnetic field of the line inductance (y LI2) and the electric field of the line capacitance (-j CV2). This energy is interchanged between the magnetic and electric fields and causes the transmission of the electromagnetic energy along the line.
In his experiments, Heinrich Hertz used end-loaded dipoles. Because of the capacitive loadings, currents could flow even at the ends of the dipole making a nearly constant current distribution possible. As explained in Chapter 2, a fluctuating current produces electromagnetic waves The current produces a changing magnetic field, the changing magnetic field produces a changing electric field, the changing electric field produces a changing magnetic field, and so on.
Being interfered with by one's own transmitter is called co-site interference. Care must be taken to assure sufficient isolation. There are many measures to be taken to mitigate co-site interference. Among those that should be considered are frequency separation, receiver selectivity, use of separate antennas and their sufficient separation, shielding, filtering transmit output, power amplifier linearity and grounding, bonding, and shielding. The worst environments for co-site interference and other forms of electromagnetic interference (EMI) include airborne and shipboard situations, particularly on military platforms.
A braid, or mesh, conductor, made of copper or aluminum, surrounds the insulation and foil shield. This conductor is normally connected to ground to create an electrostatic shield for the carrier wire. Together with any foil shield, the earthed braid protects the carrier wire from electromagnetic interference (EMI) and radio frequency interference (RFI). You should carefully note that the braid and foil shields provide good protection against electrostatic interference when earthed correctly, but little protection against magnetic interference.
- the nature of the materials Table 6.1 presents numerical values of the attenuation of electromagnetic waves obtained for different types of materials at the 17 and 60 GHz frequencies and in horizontal (H) and vertical (V) polarisations respectively (Fiacco 1998). Further detail on this subject can be found in the final
The part of the electromagnetic spectrum referred to as the microwave region loosely includes the range 1-300 GHz for practical purposes. Microwaves can be used for almost any of the applications for which the lower frequencies are used, but some of the advantages of the shorter waves make them more applicable for certain purposes. The two main advantages of microwaves are that the energy can be focused into narrow beams and that large signal bandwidths are possible.
Electromagnetic waves propagate in a vacuum with the speed of light, c 299,792,458 m s or about 3 X 108 m s. The electric and magnetic fields of a plane wave oscillate in phase and are perpendicular to each other and to the direction of propagation. The frequency of oscillation is f, and the wavelength is A c f Electromagnetic waves also may be considered to behave like particles of zero rest mass. The radiation consists of quanta, photons that have an energy of W hf where h 6.6256 X 10 34 Js is Planck's constant. There are many sources of electromagnetic radiation. Accelerating charges produce electromagnetic radiation, as when charges decelerating in an electric field produce bremsstrahlung and charges orbiting in a magnetic field produce synchrotron radiation. The random thermal motion ofcharged particles in matter produces thermal radiation. Atoms and molecules emit spectral line radiation as their energy level changes. The radiation generated by oscillators and emitted by...
In a wired local-area network (LAN), the network ports and cables are mostly contained inside a building. Therefore, a hacker must defeat physical security measures, such as security personnel, identity cards, and door locks, to be able to physically access the LAN. However, the penetration capability of electromagnetic waves exposes the datatransmission medium of a wireless LAN (WLAN) to potential intruders (Potter & Fleck, 2003).
The use of electromagnetic waves as the medium instead of cables presents many technical challenges. To begin with, the available radio band is limited, and most ranges are licensed by governments across the world. This restriction forces various wireless devices to crowd into the same unlicensed bands. Also problems inherent in radio communication, such as noise, interference, and security issues, also affect wireless networks. The implication is that, despite its growing importance in recent years, the wireless network is not going to replace the wired network. It is most
As discussed in Chapter 3, computers and other telecommunication devices use signals to represent data. These signals are transmitted from one device to another in the form of electromagnetic energy. Electromagnetic signals can travel through a vacuum, through air, or through other transmission media. Electromagnetic energy, a combination of electrical and magnetic fields vibrating in relation to each other, includes power, voice, radio waves, infrared light, visible light, ultraviolet light, and X, gamma, and cosmic rays. Each of these constitutes a portion of the electromagnetic spectrum (see Figure 5.0-1). Not all portions of the spectrum are currently usable for telecommunications, however, and usable media are limited to a few types. Voice-band frequencies are generally transmitted as current over metal cables, such as twisted-pair, or coaxial cable. Radio frequencies can travel through air or space, but require specific transmitting and receiving mechanisms. Visible light, the...
Everyone is familiar with radio because of the radios in our cars. They work by receiving transmitted electronic waves and converting them into sound. RF communications is the same, but the radios use both transmit and receive radio signals. These signals are low-power signals so that receiving and transmitting hubs need to be placed close together. All radio communications share one immutable resource, the electromagnetic spectrum. Two basic pieces of knowledge are needed to put RF communications in perspective. One has to do with the physics of radio communications and the other with the regulation of radio communications. RF telecommunications are governed by
Almost all 'Wireless' specialists will agree that the lack of standards was one of the main factors that held up the progress of WLANs. In 1997 the IEEE adopted the first Wireless LAN (WLAN) standard, IEEE Std 802.11-1997. There are three wireless LAN (WLAN) types, each of which uses a different part of the electromagnetic spectrum Infrared, Microwave, and Spread Spectrum Technology (SST). Each solution has its unique advantages and disadvantages associated with the nature of its electromagnetic spectrum frequency. In 1999 a revision was made to this standard. This IEEE standard defines a medium access control (MAC) sublayer, MAC management protocols and services, and also three physical layers. The three physical (PHY) layers are comprised of a direct sequence spread spectrum (DSSS) radio, a frequency hopping spread spectrum (FHSS) radio, both of which operate in the 2.4 GHz band, and an infrared (IR) baseband. The 802.11 standard describes these layers as providing operations of 1...
The Institute of Electrical and Electronics Engineers (IEEE) has developed a set of wireless standards that are commonly used for local wireless communications for PCs and laptops called 802.11. Currently, 802.11b and 802.11a are two basic standards that are accepted on a wider scale today. These standards are transmitted by using electromagnetic waves. Wireless signals as a whole can either be radio frequency (RF) or infrared frequency (IR), both being part ofthe electromagnetic spectrum (Boncella, 2002). Infrared (IR) broadcasting is used for close range communication and is specified in IEEE 802.11. The IR 802.11 implementation is based on diffuse IR which reflects signals off surfaces such as a ceiling and can only be used indoors. This type of transport is seldom used.
Satellite communications are nothing new. For years, you could hook up a Very Small Aperture Terminal (VSAT) system and buy time on a satellite. The VSAT can deliver up to 24 Mbps in a point-to-multipoint link (for example, a multicast) and up to 1.5 Mbps in a point-to-point link. These systems are fine for predictable and quantifiable data transmission, but if you need to conduct business on the fly, they can be a problem. New techniques are required to handle the on-demand usage. Primary among them are more tightly focused beams and digital signal technology, which together can increase frequency reuse (and thereby increase bandwidth) and reduce dish size from meters to centimeters. Accordingly, you also need a large unused chunk of the electromagnetic spectrum.
The IEEE defines electromagnetic compatibility (EMC) as The requirements for electromagnetic emission and susceptibility dictated by the physical environment and regulatory governing bodies in whose jurisdiction a piece of equipment is operated. We'll call electromagnetic emission (EMI) RFI. It just means the level of RF interference caused by a certain piece of equipment such as a microwave terminal. Susceptibility deals with how well a piece of equipment can operate in an RFI environment. EMC can be a real headache for a microwave engineer.
Radio telephony was first demonstrated in 1915 when an analog voice signal was modulated onto a radio wave. Because electromagnetic waves propagate over a wide geographical area, they are ideally suited for a radio broadcasting service where information from a source or station is transmitted to a community of receivers that is within range of the signal. The economics of this type of communication dictate that the cost can be high for the station equipment but that the cost of the receivers must be low so that the service can become available to a large number of users. Commercial broadcast radio was introduced in the early 1920s, and within a few years the service was used by millions of homes.
Higher-level packets and some large management frames may need to be broken into smaller pieces to fit through the wireless channel. Fragmentation may also help improve reliability in the presence of interference. The primary sources of interference with 802.11 LANs are microwave ovens, with which they share the 2.4-GHz ISM band. Electromagnetic radiation is generated by the magnetron tube during its ramp-up and ramp-down, so microwaves emit interference half the time.121
There were also disadvantages to the implementation of computer control. Electromechanical exchanges were rugged and worked well over wide ranges of temperature, humidity, and vibration. They could withstand high levels of EMI (electromagnetic interference). Not so with solid-state devices. None of the above is valid. Microprocessors, computers, and other solid-state control elements are much more finicky, with heat being a particular problem. In fact it is advisable to keep these devices under air conditioning, vibration, EMI, and humidity control. EMI may turn out to be the worst of them all. Many depend on chopper power supplies which emit high levels of RF noise, often related to the chopping rate.
The next two chapters provide more depth and detail by outlining a complete telecommunication system. When the transmitted signal is passed through the air using electromagnetic waves, it must take the form of a continuous (analog) waveform. A good way to understand such analog signals is via the Fourier transform, and this is reviewed briefly in Chapter 2. The five basic elements of the receiver will be familiar to many readers, and they are presented in Chapter 3 in a form that will be directly useful when creating Matlab implementations of the various parts of the communications system. By the end of the second layer, the basic system architecture is fixed the ordering of the blocks in the system diagram has stabilized.
The creation of an electromagnetic field is easy to understand qualitatively with the aid of Maxwell's equations. Let us consider a current loop with a changing current. The changing current creates a changing magnetic field (IV) the changing magnetic field creates a changing electric field (III) the changing electric field creates a changing magnetic field (IV) and so on. Figure 2.2 illustrates the creation of a propagating wave. Maxwell's equations form the basis of radio engineering and, in fact, of the whole of electrical engineering. These equations cannot be derived from other laws they are based on empirical research. Their validity comes from their capability to predict the electromagnetic phenomena correctly. Many books deal with fundamentals of the electromagnetic fields, such as those listed in 1 8 .
Twisting the pairs of wires minimizes crosstalk between pairs. The twists also help deal with electromagnetic interference (EMI) and radio frequency interference (RFI), as well as balancing the mutual capacitance of the cable pair. The performance of a twisted-pair cable can be influenced by changing the number of twists per meter in a wire pair. Each of the pairs in a 4-pair category 5 cable, for example has a different twist rate to reduce the crosstalk between them.
Electromagnetic fields are vector quantities, which have a direction in space. The polarization of a plane wave refers to this orientation of the electric field vector, which may be a fixed orientation (a linear polarization) or may change with time (a circular or elliptical polarization).
If electromagnetic waves are propagating in media of low loss the effect of the medium can be characterized with a complex permittivity, imaginary part of which is significantly smaller than its real part. Loss due to the electrically neutral molecules of the air can be characterized like that and is usually negligible. Electrically asymmetrical molecules cause higher loss it is reasonable to characterize them more precisely.
The troposphere is the lowest 10 km part of the atmosphere. The inhomogenities within the common beam volume of the transmit and receive antenna act as sources scattering the electromagnetic waves into all directions of space. Thus components will be generated establishing coupling between transmitter and receiver. Tropospheric links are based on this weak coupling mechanism 22.214.171.124 . The first troposcatter link has been put into operation in 1953.
Meteorites entering the atmosphere are glowing and producing an ionized band lasting from a few tenth of a second up to a few seconds at a hight of 80 to 120 km. Depending on the density of the charged particles, this burst will either scatter or reflect electromagnetic waves in the 40 to 100 MHz range, allowing communication to be established over distances of 400 to 2000 km 126.96.36.199 . This phenomenon was discovered in 1935, and later on, between 1950 and 1975, intensive development work has been carried out in order to determine the properties of these radio channels. The first meteor-burst system started operation in Canada, in the year 1953, and in 1967, the ANOTEL (Snowpack Telemetry) system in Alaska was put into operation. This system was used in an uninhabited area for transmitting sensing data from 500 remote stations to a central station.
Relating to the user's health protection, there is a Hungarian decree 8.4.11 which makes a set of standards of safety character, e.g. protection against the athmospheric effects, effects of power lines, obligatory. This set of standards should be extended in the future, e.g. by those related to safety of devices using laser radiaton (see Hungarian standard MSZ EN 60825-1 2000), by standards concerning issues of the health protection against the electromagnetic radiation which issue is already dealed with in a Hungarian decree 8.4.12 and so on.
In the area of health, for example, there have been numerous allegations over the years about the dangers of excessive use of ICTs. Electromagnetic fields from antennas and mobile phones are alleged to emit radiation that can cause cancer and other illnesses.6 Other studies have shown links between extensive computer use and physical ailments such as poor eyesight due to flickering and reflection on the screen and muscular pain caused by static and poor posture. Excessive movement of the wrist and hand have been said to lead to inflammation of the tendon and carpal tunnel syndrome.7 Another modern-day illness related to increased use of computers and the Internet is infostress related to an overwhelming load of information.8 Excessive use of modern ICTs can even be deadly. In the Republic of Korea, where online game addiction has become a serious problem, a teenager died at his terminal in an Internet caf after three days of continuous playing.
As in all networks, data transmission in wireless networks takes place over a network medium. The medium in this case is free space, and the transmission method is a type of electromagnetic radiation. In order to be well-suited for deployment on mobile networks, the medium must encompass an extensive area in order to facilitate the movement of the clients throughout that area.
The highest layers of the Earth's atmosphere are called the ionosphere, because they contain plasma, which is ionized gas (free electrons and ions). The ionosphere extends from 60 to 1,000 km. Below 60 km the ionization is insignificant because the solar ionizing radiation is getting weaker due to absorption in the higher layers, and because recombination of plasma is fast due to high density of molecules. Above 1,000 km the density of molecules is too low for a significant phenomenon. It is possible to distinguish different layers in the ionosphere, as shown in Figure 10.15 they are called D, E, F1, and F2 layers. The electron density and the height of these layers depend on the solar activity, on the time of day and season, and on the geographical location. During night the D layer nearly disappears, and the F1 and F2 layers merge together. The highest electron density is about 1012 electrons m3, and it can be found at daytime at the altitude of about 250 to 400 km in the F2 layer.
An optical waveguide directs the propagation of an electromagnetic field through a nearly lossless doped silica medium. We refer to the propagating field as the lightwave. The field obeys Maxwell's equations. Because the changes in refractive index n(x) are small, Maxwell's equations reduce to a scalar wave equation for the transverse electric field. At a single wavelength A and polarization, the scalar wave equation is
Fiber has been used in North America since the 1960s. The carriers recognized the limitation of the copper cable they had been installing for years. First, the copper was very expensive. Moreover, it was also highly susceptible to noise and interference (both Radio Frequency Interference RFI and Electromagnetic Interference EMI ). The carriers tried to get more capacity on the wires to make the cable plant more efficient and cost effective. However, modulation techniques left the carriers wanting more. Most of the cabling systems were installed by using some form of analog transmission system. In the late 1950s and into the 1960s, the Bell System developed a digital modulation technique, which could take advantage of the limited bandwidth and capacities of the copper cables.
Before going any further into the DS-1, it may be appropriate to review the modulation technique used to create the digital signal. When DS-1 was first created, it was designed around converting analog voice communications into digital voice communications. To do that, voice characteristics were analyzed. What the developers learned was that voice operates in a telephony world in a band-limited channel operation. The normal voice will produce both amplitude and a frequency change ranging from 100 to approximately 5,000 times a second. These amplitude and frequency shifts address normal voices. However, the telephone companies decided long ago that carrying true voice would be too expensive and would not provide any real added value to the conversation. They then determined that the normal conversation from a human actually carries the bulk of the information when the frequency and amplitude shifts vary between 100 and 3,300 times per second. Armed with this information, the developer...
Twisting wire pairs cancels out radiated energy from current flowing in any one wire by the radiated energy from the same current flowing back in the return wire of the same pair. Radiated energy is called Electromagnetic Radiation (EMR). Twisting effectively and inexpensively minimizes crosstalk between adjacent pairs in a multi-pair cable. Twisting also makes the wire pairs less susceptible to external noise. The noise is coupled equally into each wire in a pair, causing the noise to cancel out when the wires are properly terminated. At voice frequencies, each pair appears to be balanced, e.g., equal electrical energy is emitted from each wire within the pair to any point outside the pair of wires.
In the past, two parallel flat wires were used for communication. However, electromagnetic interference from devices such as a motor can create noise over those wires. If the two wires are parallel, the wire closest to the source of the noise gets more interference and ends up with a higher voltage level than the wire farther away, which results in an uneven load and a damaged signal (see Figure 5.1-4).
The Nature of Light Light is a form of electromagnetic energy. It travels at its fastest speed in a vacuum 300,000 kilometers second (approximately l86,000 miles second). The speed of light depends on the density of the medium through which it is traveling (the higher the density, the slower the speed).
The electromagnetic disturbance created by the transmitter is propagated by the transmitter antenna and travels at the speed of light as described in Chapter 2. It is evident that, if the electromagnetic wave encounters a conductor, a current will be induced in the conductor. How much current is induced will depend on the strength of the electromagnetic field, the size and shape of the conductor and its orientation to the direction of propagation of the wave. The conductor will then capture some of the power present in the wave and hence it will be acting as a receiver antenna. However, other electromagnetic waves emanating from all other radio transmitters will also induce some current in the antenna. The two basic functions of the radio receiver are
In 1864, James Maxwell (1831-1879), a Scottish physicist, produced his theory of the electromagnetic field which predicted that electromagnetic waves can propagate in free space at a velocity equal to that of light 9 . Experimental confirmation of this theory had to wait until 1887 when Heinrich Hertz (1857-1894) constructed the first high-frequency oscillator. When a voltage was induced in an induction coil connected across a spark gap, a discharge would occur across the gap setting up a damped sinusoidal high-frequency oscillation. The frequency of the oscillation could be changed by varying the capacitance of the gap by connecting metal plates to it. The detector that he used consisted of a second coil connected to a much shorter spark gap. The observation of sparks across the detector gap when the induction coil was excited showed that the electromagnetic energy from the first coil was reaching the second coil through space. These experiments were in many ways similar to those...
The structure of the coaxial cable ensures that, at normal operating frequencies, the electromagnetic field generated by the current flowing in it is confined to the dielectric. Radiation is therefore severely limited. At the same time, the outer conductor (normally grounded) protects the cable from extraneous signals such as noise and cross-talk.
EMC breaks down into two subsets (1) emanation and (2) susceptibility. Nearly all electronic devices emanate. In other words, they produce measurable RF energy, what we sometimes call electromagnetic interference (EMI) or radio-frequency interference. The path of this RF energy can be conducted or radiated. How much RF energy is produced and the harm it can do to that same device or other nearby devices are vital to good system equipment design. The PC (personal computer) is a notorious em-anator.
Radio communication in free space is possible because, when an alternating current flows in a conductor, part of the energy is lost in the form of electromagnetic radiation into free space. When the frequency of the current is low, the radiation ''loss'' is very small, but as the frequency increases, substantial losses can occur. In designing an antenna the object is to construct a structure which will maximize the radiated energy in a given direction or over a geographic area. where 1 is the wavelength in meters and f is the frequency in hertz. Electromagnetic radiation has two vectors the E and H which are orthogonal to each other and to the direction of propagation. If the transmission line is bent at right angles as shown in Figure 2.51(b), the structure is called a dipole and it has the current distribution shown. Since the oppositely directed currents are now far apart, they do not counteract each other and hence the dipole is a good radiator of electromagnetic waves.
The transmission system involves analog transmission (voice communication) and digital transmission. The analog signals are characterized by frequency, amplitude and phase. In analog transmission system, signals propagate through the medium as continuously varying electromagnetic waves. The medium for an analog transmission may be twisted pair cable, coaxial cable, optical-fiber cable, microwave radio and satellites. The analog signals are subjected to deterioration due to attenuation and noise addition in the channel. Hence amplifiers, filters and necessary circuits are added in transmission system to upgrade the analog signal. Fig. 3.8 shows the voice signal and digital signals.
Maxwell's equations relate the fields (E and H) and their sources (p and J) to each other. The electric field strength E and the magnetic flux density B may be considered the basic quantities, because they allow calculation of a force F, applied to a charge, q, moving at a velocity, v, in an electromagnetic field this is obtained using Lorentz's force law
When evaluating the suitability of a particular medium to a specific application, five factors should be kept in mind cost, speed, attenuation, electromagnetic interference, and security. Electromagnetic interference (EMI). The susceptibility of the medium to external electromagnetic energy inadvertently introduced onto a link that interferes with the intelligibility of a signal. Familiar effects of EMI are static (audio) and snow (visual).
The electromagnetic spectrum spreads over the frequency range from near zero to 1023 Hz. In this very wide band take place the radio waves and also the light. The radiospectrum is a part of the electromagnetic spectrum at which frequencies the waves can generated, radiated and received efficiently and therefore will be applicable for different radio services. The part of the electromagnetic spectrum below 3000 GHz concerns radio waves. The radio waves propagate of radiation from an antenna without man-made ducting.
The 100BASE-T Ethernet LAN is also known as Fast Ethenet. As indicated by the designation, 100BASE-T Ethernet operates at a speed of 100 Mbps using twisted-pair wire. The computers are connected to a hub or a switch in a star topology, and the distance of the twisted pairs is limited to 100 meters. Operating 100 Mbps on UTP is challenging, and so three options for doing so were developed, one for category 3 UTP, one for shielded twisted pair, and one for category 5 UTP. One problem with extending the 10BASE-T transmission format is that Manchester line coding is inefficient in its use of bandwidth. Recall from the section on line coding that Manchester coding pulses vary at twice the information rate, so the use of Manchester coding would have required operation at 200 Mpulses second. Another problem is that higher pulse rates result in more electromagnetic interference. For this reason, new and more efficient line codes were used in the new standards.
In Table 1-1, there are three bands labeled ISM, which is an abbreviation for industrial, scientific, and medical. ISM bands are set aside for equipment that, broadly speaking, is related to industrial or scientific processes or is used by medical equipment. Perhaps the most familiar ISM-band device is the microwave oven, which operates in the 2.4-GHz ISM band because electromagnetic radiation at that frequency is particularly effective for heating water.
Chapter 3 examines some of the different types of transmission media used for physically conveying signals from one point to another. The approach taken will be to explain the fundamental method of operation of each of these transmission media types, introduce the various system components and discuss the application for each type. Some of the main bearer design considerations will be discussed to enable the reader to make an informed decision as to which type of media to use for a particular application. The discussion will commence with systems guided over a physical bearer namely twisted pair and coaxial copper, fiber-optic cables and the power distribution system. The discussion will then move on to wireless systems namely microwave radio systems, satellite systems and infra-red transmission, which require no specific bearer and radiate their signals as electromagnetic waves. The emphasis in this chapter is on the fundamental transmission bearer systems, Chapter 6 discusses the...
Fiber-optic communication uses light signals and so transmissions are not subject to electromagnetic interference. Fiber-optic cables act as a waveguide for light, with all the energy guided through the central core of the cable. The light is guided due to the presence of a lower refractive index cladding surrounding the central core. None of the energy in the signal is able to escape into the cladding and no energy is able to enter the core from any external sources. The composition of the cable is shown in Figure 3.15.
Radio encompasses the electromagnetic spectrum in the range of 3 kHz to 300 GHz. In radio communications the signal is transmitted into the air or space, using an antenna that radiates energy at some carrier frequency. For example, in QAM modulation the information sequence determines a point in the signal constellation that specifies the amplitude and phase of the cosine wave that is transmitted. Depending on the frequency and the antenna, this energy can propagate in either a unidirectional or omnidirectional fashion. In the unidirectional case a properly aligned antenna receives the modulated signal, and an associated receiver in the direction of the transmission recovers the original information. In the omnidirectional case any receiver with an antenna in the area of coverage can pick up the signal.
Single-mode fibers operate similar to microwave RF wave-guide. The wave-guide's physical dimensions approach the wavelength of the electromagnetic radiation being transmitted. This prevents microwaves from traveling through the wave-guide to escape the wave-guide. The same principal is used to construct see-through door panels on microwave ovens. The wire mesh in the door prevents the microwaves from escaping the microwave oven.
Electromagnetic fields are formed as voltage and current waves travel down a transmission line. As propagation frequency increases a portion of the associated wavelength becomes comparable to the geometry of the transmission line dimensions. As the frequency increases, different types of propagation modes appear. The principal mode is the one which can carry energy at all frequencies. Higher modes are those that propagate only above a definite frequency range, and the point at which these frequencies start to propagate is called the cut-off frequency for that particular mode. An analogy is shown in Fig. 7.6.
Mobile networking is a subject that is becoming increasingly important as portable devices such as personal digital assistants (PDAs) and notebook computers are becoming more powerful and less expensive, coupled with people's need to be connected whenever and wherever they are. The link between the portable device and the fixed communication network can be wireless or wired. If a wireless link is used, the device can utilize a radio or infrared channel. Of these two alternatives, radio channels can traverse longer distance without the line-of-sight requirement, but introduce electromagnetic interference and are often subject to federal regulations (e.g., Federal Communication Commission, or FCC). Infrared channels are often used in shorter distances. A wireless connection enables a user to maintain its communication session as it roams from one
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