Radiation from Antennas

THEORY OF ELECTROMAGNETIC WAVE PROPAGATION, Charles Her-ach Papas. Graduate-level study discusses the Maxwell field equations, radiation from wire antennas, the Doppler effect and more, xiii + 244pp. 5b x 8b. 

In order to calculate the coefficient of scattering we start from the familiar formulae for the radiation from a dipole antenna. According to Hertz, the field of a dipole of moment p is given by... 

Plane-polarized elceiromngneiic radiation is produced bv ccriain radiani energ sources, r )r example, ihe radio sa es cmanaling from an antenna and the micrt)wavcs pioklucedby a klystron lube are both plane polarized. Visible and ultraviolet radiation from relaxation of a single excited atom or molecule is also polarized. hut the heam from such a source has no net... 

To solve this problem it is necessary to use zero balance radiometer with compensation of reflections between antenna and human body tissue. This principle is realized in most modem microwave radiometers An overview of microwave radiometry is given in a recent publication The balance multi-fiequency microwave radiometer has also been described These researchers used 5 frequencies in calculating the temperature profile in the brain. It is very important to use multi-frequency radiometer in order to visualize the temperature inside body. But the increase in tile munber of fiequencies increases the size and the weight of the radiometer and decreases the noise immunity of the device. The radiation from the human body is very small. So the noise immunity is one of the critical parameters of the microwave radiometer. 

Furthermore, in Chapter 2 we presented the theory for the RCS of antennas in general. In particular, it was shown that the scattering from antennas can be decomposed into two components, namely the antenna mode component being proportional to the square of the reflection coefficient observed at the antenna terminals and another called for the residual component (also earlier denoted structural). Although the precise definition of the second was somewhat illusive, we nevertheless demonstrated that it was equal to zero for large fiat apertures with uniform illumination. The most prominent member of that family was without a doubt the antenna array. In fact it has a very unique position in the world of radiators. 

To summarize No energy is radiated from the infinite array, only from the finite. Thus, a test antenna located a reasonably large distance from the array will pick up a signal only from the finite array, not the infinite. Furthermore, from the law of reciprocity we may conclude that a signal from the test antenna cannot produce a surface wave on the infinite array but can readily do so on the finite array. 

Phycobilisomes (PBsomes) constitute the light-harvesting antennae of cyanobacteria. Most of them are made up of a central three-cylinder core from which six rods radiate (1). However, exceptions have been described i) Synechococcus sp. PCC 6301 and 7942 have hemidiscoidal PBsomes, but the six rods radiate from a core made up of only two cylinders (2) ii) according to the observed ratio of rod phycobiliproteins (PB) versus core PBs, as well as to electron microscopy studies, Phormidium sp. PCC 7376 seems to have PBsomes resembling more closely the hemi-ellipsoidal tyjpt described for most of the rhodophytes (3) iii) finally, Gloeobacter violaceus, which does not have classical thylakoid membranes, harbors very peculiar rod-shaped PBsomes directly attached to the cytoplasmic membrane (4). 

In the case of an aperture such as that shown in Fig. 13.1(e), the primary currents are flowing on the wave-guide and horn surfaces and are too difficult to find. In this case, the standard practice is to introduce what are known as equivalent currents. For the rectangular aperture at the mouth of the horn, equivalent electric and magnetic currents can be found from the equivalence principle so that the radiation from the equivalents is the same as from the primary currents. In any event, the current must be known before proceeding with the analysis of an antenna. 

It has been recognized that one of the fundamental properties of frequency-independent antennas is their abihty to retain the same shape under certain scahng transformations. It has been demonstrated that this property of self-similarity is also shared by many fractals (Mandelbrot, 1983). This commonality has led to the notion that fractal geometric principles be used to provide a natural extension to the traditional approaches for classification, analysis, and design of frequency-independent antennas (D.H. Werner and P.L.Werner). This theory allows the classical interpretation of frequency-independent antennas to be generalized to include the radiation from structures that are not only self-similar in the smooth or discrete sense but also in the rough sense. 

At certain locations, generally near high-power broadcasting antennas or near high-power RF sealing or welding equipment, fields are Hkely to exceed the exposure guidelines. As NIER power radiates from a source, the spherical surface over which the wave spreads increases as the square of the distance from the source. This leads to the so-called inverse square law by which it is known that, in the far field of an antenna, the power density decreases rapidly. Calculation of RF exposure conditions rehes on this fact from basic physics, and the incorporation of certain conservative assumptions set forth by the FCC in its... 

What is radio astronomy Radio telescopes are dish antennas that measure the intensity of radiation from space at centimeter wavelengths. The goal of radio astronomy is to use these signals to learn about the source that emits them stars, clouds of interstellar gas, and even objects that would be invisible to optical telescopes. 

When a current source is located within a clad fiber of arbitrary profile, the determination of the radiation field is extremely complicated. However, if the fiber is weakly guiding the determination is greatly simplified [2]. It is intuitive that when the variation in the refractive-index profile is small, the source radiates as if it were located in a virtually uniform, infinite medium of refractive index equal to the cladding index n j. The problem is then analogous to the radiation from an antenna in free space. Consequently, we can borrow from standard antenna theory, and couch the solution to radiation from the weakly guiding fiber in terms of the electromagnetic vector potential A [3-5]. 

To calculate the power radiated from the bend, it is sufficient to use an approximate model of the fiber consisting of a current-carrying antenna of ir nitesimal thickness which radiates in an infinite medium of index equal to the cladding index n, i.e. the free-space approximation of Section 21-8. The far-field formulae of antenna theory, as expressed by Eq. (21-21), can then be applied to calculate the radiation, once the current on the antenna is specified. 

The ratio P /P(0) is the fraction of power radiated from the entire loop in Fig. 23-2(a), and ignores attenuation of local-mode power along the antenna. 

In Section 22-5 we determined the attenuation of the fundamental mode on a weakly guiding, step-profile fiber due to radiation from a sinusoidal perturbation of the interface, using free-space antenna methods and correction factors. Here we consider the situation when the radiation field is well approximated by a single leaky mode, which can be realized by having an on-axis sinusoidal nonuniformity of the form of Eq. (22-14). The azimuthal symmetry ensures that only HEi leaky modes are excited. Further, the direction of radiation should coincide with the direction of the leaky-mode radiation [23]. If we represent the nonuniformity and the incident fundamental-mode fields by the induced current method, as in Section 22-5, the direction condition is satisfied by setting C = in Eq. (24-43), whence... 

In this first set of examples, we determine radiation from sources in free space , using the free-space radiation modes, and compare the results with the antenna methods of Chapter 21. 

There is also a further EM enhancement effect to consider which is caused by the oscillating molecular dipole at position r inducing a dipole (actually higher multipoles are also induced) in the metal sphere. Thus, the metal sphere acts as an antenna for the near field of the oscillating molecular dipole, and the emitted Raman radiation from the molecule at frequency (Og (Raman-shifted frequency) is then enhanced by the presence of the metal particle. The dipole moment of the entire system, the so-called emission dipole of the molecule plus metal particle system, includes the effect of both the enhanced... 

Plastics will continue to be required in space applications from rockets to vehicles for landing on other planets. The space structures, reentry vehicles, and equipment such as antennas, sensors, and an astronaut s personal communication equipment that must operate outside the confines of a spaceship will encounter bizarre environments. Temperature extremes, thermal stresses, micrometeorites, and solar radiation are sample conditions that are being encountered successfully that include the use of plastics. 

Radome Also called radiation dome. It is a cover for a microwave antenna used to protect the antenna from the environment on the ground, underwater, and in the air (aircraft nose cone, etc.). The dome is basically transparent to electromagnetic radiation and structurally strong. Different materials have been used such as wood, rubber-coated air-supported fabric, etc. The most popular is the use of glass fiber-TS polyester RPs. The shape of the dome, that is usually spherical, is designed not to interfere with the radiation. 

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