File Name: electromagnetic waves and antennas .zip
Exercises I2 throughI5 pertain to the classical picture of electromagnetic radiation. Classical Electromagnetic Radiation-Jerry Marion Classical Electromagnetic Radiation, Second Edition focuses on the classical electrodynamics with emphasis on radiation problems and the wave attributes of the electromagnetic field. Radiation is energy.
Radiation resistance is that part of an antenna 's feedpoint electrical resistance that is caused by the radiation of electromagnetic waves from the antenna. The transmitter generates a radio frequency alternating current which is applied to the antenna, and the antenna radiates the energy in the alternating current as radio waves. Because the antenna is absorbing the energy it is radiating from the transmitter, the antenna's input terminals present a resistance to the current from the transmitter. Unlike other resistances found in electrical circuits, radiation resistance is not due to the opposition resistivity of the material of the antenna conductors to electric current; it is a virtual resistance due to the antenna's loss of energy as radio waves.
Engineering Electromagnetics pp Cite as. After discussing wave propagation, it is time we go back and discuss the sources of the waves. Recall that our whole discussion of waves was based on the solution to the source-free wave equation. However, the sources of waves are extremely important.
To transmit power, we must first generate the waves at the proper level and frequencies and, second, we must couple the energy into the appropriate domain. This coupling is done by what we call an antenna. In the following sections, we will discuss the basic principles of antennas starting with the elementary electric and magnetic dipoles. Then, we extend these to other important antennas and discuss the relations between receiving and transmitting antennas, principles of design of antenna systems, as well as some important applications of antennas.
Radiation is the process of emitting energy from a source. We know from experience that radiation at various frequencies is different. At low frequencies, we talk about electromagnetic waves. In the visible domain, the emission is in the form of light.
At still higher frequencies, the emission may be ultraviolet or X-ray radiation. Each of these is an electromagnetic wave, but the properties of the wave change with frequency. For example, low-frequency electromagnetic waves are not visible whereas X-rays easily penetrate through our bodies.
We also know that X-rays can be damaging to cells and ultraviolet rays are known to harm our eyes and, in some cases, to cause skin cancer. This observation raises more questions than it answers. For example, we may ask: If X-rays are dangerous, why not visible light or microwaves or, indeed, any electromagnetic wave? Or perhaps they are?
Even more important is to ask ourselves what makes X-rays dangerous while other forms of electromagnetic radiation are not? What, then, is the difference between the various types of radiation? This energy, also known as the quantum of radiation , indicates the relative energy in different ranges of radiation. In the visible range 4. In their lowest range, X-rays are at least 10 times more energetic than visible light.
For this reason, ultraviolet light, which overlaps part of the X-ray domain, is considered to be harmful. On the other hand, microwave photon energy is at least 4 orders of magnitude lower than the lowest X-ray energy. This distinction between low- and high-energy domains is sometimes made on the basis of the ability of the various emissions to ionize materials through which they pass.
Low-frequency low-energy radiation is called nonioni z ing and includes all frequencies up to the low ultraviolet. This is in contrast with, for example, radioactive radiation, which is ionizing. In this chapter, while using the term radiation, this radiation should be understood as nonionizing radiation. Are there any other effects you can think of that might change your view as to safety of either radiation?
From these considerations, it would appear that the radar radiation is safer than solar radiation. Also, clothing shields visible light although less effective in the ultraviolet region and beyond but does not shield lower-frequency waves like the radar in question.
What is an antenna then? Quite simply, it is any structure that can radiate electromagnetic energy into a medium. For example, an infinite sheet of current is an antenna that generates a plane wave. The principle is quite simple: An antenna must provide a time-dependent current which, in turn, generates a magnetic and an electric field.
When time-dependent electric and magnetic fields exist, power is generated and propagated. Although this is true in general, it is not immediately obvious why one structure may serve as an antenna while others cannot, and, in fact, how an antenna radiates is still not entirely clear. Common antennas. It is obvious that we have some explaining to do, although, once done, the answer looks surprisingly simple. We will start with the simplest of structures, the elemental dipole radiator, and build on this to define more complex and more practical antennas.
One is an electric dipole, the other a magnetic dipole. Both may be used as radiators and we will discuss both. One charge is negative, the other positive. The point charges are time dependent. We will assume a sinusoidal time dependency, but any other time dependency may be used. The two point charges are connected through a thin, conducting wire. A time-dependent current can now flow back and forth between the point charges. Note that the total charge at any time must be zero, as required by the law of conservation of charge.
So, what can we do? However, the solution to the wave equation is quite complicated. Instead of actually solving the wave equation, we rely here on a physical argument that will allow us to obtain the correct solution without the need of actually solving a wave equation. These equations give the electric and magnetic field intensities of the Hertzian dipole everywhere in space and for all conditions. These expressions can now be used to calculate anything else we need, such as direction of propagation, power density, and the like.
However, the expressions are rather complicated, and we will seek to simplify them before applying them to practical antennas. Thus, we can define three distinct domains: One is the domain of small values of R , which we will call the near-field domain or the Fresnel zone , and one of large values of R , which is called the far-field domain or the Fraunhofer zone. The third domain is an intermediate domain where neither assumption holds, called the inductive zone.
The inductive zone is of little importance and, therefore, we will not discuss it separately other than to say that it is a transition zone and, if necessary, it may be analyzed using the above general equations.
Open image in new window. Thus, we obtain a static-like solution or what we call a quasi-static solution. In other words, the electric field intensity in the near field of a Hertzian dipole behaves like the electrostatic dipole. For this reason, the near field of the dipole is also called the electrostatic field even though it is not static.
The same can be said about the magnetic field intensity in Eq. No propagation effects can be seen in either Eq. The approximate solution in the near field shows no wave behavior because the wave effects are small compared to the electrostatic field and these effects were neglected by the approximations used to reach the results in Eqs.
Thus, in the near field, the dipole does not radiate, as can also be seen from the Poynting vector. Because the magnetic field intensity in Eq. Therefore, in the near field, there is only storage of energy.
Also, since the electric field intensity is much larger than the magnetic field intensity, the stored electric energy is higher than the stored magnetic energy and dominates in the near-field domain.
This means the dipole in the near field is essentially capacitive in nature. The electric and magnetic field intensities in the far field are perpendicular to each other. In the far field, the wave produced by the Hertzian dipole is, in many ways, similar to a plane wave, although it is not a plane wave. However, the wave approximates a plane wave because at large distances, the spherical surface of radius R approximates a plane.
Use the general expressions to show which terms may be neglected. Taking one-tenth of this, we find the near field to extend up to about 0.
Also, the electric field intensity and the magnetic field intensity are out of phase, therefore producing no propagating waves. The electric field intensity is over four orders of magnitude larger than the magnetic field intensity. This is the intrinsic impedance of free space. Thus, in the far field, the waves behave similar to plane waves. Write the time-dependent electric and magnetic field intensities of the dipole at any point in space.
After a quarter period, the electric field intensity in the vicinity of the dipole reaches maximum whereas in the far field, the field lines close on themselves and propagate, expanding radially. The time-averaged Poynting vector in the near field of a Hertzian dipole is imaginary. The time-averaged Poynting vector in the far field of a Hertzian dipole is real and directed in the positive R direction. The various properties of antennas are defined next. These include important antenna operation parameters such as the power radiated by the antenna, its efficiency, as well as the concepts of directivity, radiation resistance, radiation patterns, radiation intensity, and gain.
To simplify discussion, we use the Hertzian dipole as an example throughout this section. However, the definitions are general and will be used throughout this chapter for other antennas. The power radiated by the dipole is proportional to the current squared and the length of the antenna squared.
It also depends on the intrinsic impedance of the medium in which the antenna radiates and is directly proportional to frequency squared inversely proportional to wavelength squared.
Thus, a very short dipole will radiate very little power and a longer dipole will radiate more power. We will see in the context of real antennas that, in general, the longer the antenna, the larger the radiated power. From this calculation, we can also see that the true Hertzian dipole, although fundamental, is not the most practical antenna to build because of the very low power it can radiate.
Radiation resistance is not the resistance of the antenna but is a characteristic quantity of the dipole described here and, indeed, of other radiators and reflects both the antenna structure and dimensions, as well as the environment. It simply indicates the power the dipole can radiate for a given current. Maximization of the radiation resistance means the antenna can radiate more power for any given current. Thus, the longer the antenna, the larger the radiation resistance and the larger the power it can radiate for a given current.
Calculate the radiation resistance in air and in Teflon and the ratio between the radiation resistance in air and in Teflon. Note that the power transmitted in Teflon is 1.
We can get a good understanding of electromagnetic waves EM by considering how they are produced. Whenever a current varies, associated electric and magnetic fields vary, moving out from the source like waves. Perhaps the easiest situation to visualize is a varying current in a long straight wire, produced by an AC generator at its center, as illustrated in Figure 1. Figure 1. This long straight gray wire with an AC generator at its center becomes a broadcast antenna for electromagnetic waves. Shown here are the charge distributions at four different times. The electric field E propagates away from the antenna at the speed of light, forming part of an electromagnetic wave.
Current interest in these areas is driven by the growth in wireless and fiber-optic communications, information technology, and materials science. Communications, antenna, radar, and microwave engineers must deal with the generation, transmission, and reception of electromagnetic waves. Novel recent developments in materials, such as photonic bandgap structures, omnidirectional dielectric mirrors, and birefringent multilayer films, promise a revolution in the control and manipulation of light. These are just some examples of topics discussed in this book. The text is organized around three main topic areas:. The text also emphasizes connections to other subjects, including digital signal processing, digital filter design and Fast Fourier Transform. Orfanidis Free?
The MATLAB toolbox is available here. The book is also available in printed form. Individual chapters are available below in PDF in 2-up format.
Propagation of electromagnetic waves with orbital angular momentum OAM is investigated in indoor environments. The OAM modes generated by circular patch array antennas are used. With proper alignment and suppressed multipath, the OAM modes can transport multiple wireless data stream at the same time.
In radio engineering , an antenna or aerial is the interface between radio waves propagating through space and electric currents moving in metal conductors, used with a transmitter or receiver. In reception , an antenna intercepts some of the power of a radio wave in order to produce an electric current at its terminals, that is applied to a receiver to be amplified. Antennas are essential components of all radio equipment.
Skip to Main Content. A not-for-profit organization, IEEE is the world's largest technical professional organization dedicated to advancing technology for the benefit of humanity. Use of this web site signifies your agreement to the terms and conditions. To facilitate scientific exploration a wide variety of systems and vehicles have been developed to operate within the shallow continental shelf region or in deep oceans. For successful underwater electromagnetic EM wave operation, knowledge is required of the wave transmission properties of seawater over all distances both short and long.
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: Cloude Published Engineering. This text should serve as an introduction to the application of electromagnetics EM, following an initial course in basic EM theory.
Хочешь посмотреть, чем занимаются люди в шифровалке? - спросил он, заметно нервничая. - Вовсе нет, - ответила Мидж. - Хотела бы, но шифровалка недоступна взору Большого Брата. Ни звука, ни картинки. Приказ Стратмора. Все, что я могу, - это проверить статистику, посмотреть, чем загружен ТРАНСТЕКСТ. Слава Богу, разрешено хоть .
16 Transmitting and Receiving Antennas on electromagnetic waves, which covers propagation, reflection, and transmission of waves.
Стратмор, глядя в темноту, произнес бесцветным голосом, видимо, уже все поняв: - Да, Сьюзан. Главный банк данных… Сьюзан отстраненно кивнула. Танкадо использовал ТРАНСТЕКСТ, чтобы запустить вирус в главный банк данных. Стратмор вяло махнул рукой в сторону монитора. Сьюзан посмотрела на экран и перевела взгляд на диалоговое окно. В самом низу она увидела слова: РАССКАЖИТЕ МИРУ О ТРАНСТЕКСТЕ СЕЙЧАС ВАС МОЖЕТ СПАСТИ ТОЛЬКО ПРАВДА Сьюзан похолодела.
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- Она показала ему другую колонку. - Видишь. - Вижу, - сказал Бринкерхофф, стараясь сосредоточиться на документе.
Она метнулась к буфету в тот момент, когда дверь со звуковым сигналом открылась, и, остановившись у холодильника, рванула на себя дверцу. Стеклянный графин на верхней полке угрожающе подпрыгнул и звонко опустился на место. - Проголодалась? - спросил Хейл, подходя к. Голос его звучал спокойно и чуточку игриво.
Она оказалась в тоннеле, очень узком, с низким потолком. Перед ней, исчезая где-то в темноте, убегали вдаль две желтые линии. Подземная шоссейная дорога… Сьюзан медленно шла по этому туннелю, то и дело хватаясь за стены, чтобы сохранить равновесие.
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Electromagnetic Waves & Antennas – S. J. Orfanidis – June 21, 4. Reflection and Transmission Propagation Matrices,Ray A. 07.06.2021 at 23:45
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Engineering Electromagnetics pp Cite as.