【正文】 Atmospheric absorption cosmic noiseUHF television。 high atmospheric noise levelLongrange navigation。 and night SW。 rainfall attenuation above 10GHz。 Intermodulation noise wireless local loop Infrared300GHz to400THz1mm to770nmLOSInfrared LANs。 international broadcasting, military munication。自動化與電氣工程英文資料與中文翻譯畢業(yè)論文 ANTENNASAn antenna can be defined as an electrical conductor or system of conductors used either for radiating electromagnetic energy or for collecting electromagnetic energy. For transmission of a signal, radiofrequency electrical energy from the transmitter is converted into electromagnetic energy by the antenna and radiated into the surrounding environment (atmosphere, space, water). For reception of a signal, electromagnetic energy impinging on the antenna is converted into radiofrequency electrical energy and fed into the receiver.Radiation PatternsAn antenna will radiate power in all directions but, typically, does not perform equally well in all directions. A mon way to characterize the performance of an antenna is the radiation pattern, which is a graphical representation of the radiation properties of an antenna as a function of space coordinates. The simplest pattern is produced by an idealized antenna known as the isotropic antenna. An isotropic antenna is a point in space that radiates power in all directions equally. The actual radiation pattern for the isotropic antenna is a sphere with the antenna at the center. However, radiation patterns are almost always depicted as a twodimensional cross section of the threedimensional pattern. The pattern for the isotropic antenna is shown in distance from the antenna to each point on the radiation pattern is proportional to the power radiated from the antenna in that shows the radiation pattern of another idealized antenna. This is a directional antenna in which the preferred direction of radiation is along one axis.The actual size of a radiation pattern is arbitrary. What is Important is the relative distance from the antenna position in each direction. The relative distance determines the relative power. To determine the relative power In a given direction, a line is drawn from the antenna position at the appropriate angle, and the point of intercept with the radiation pattern is determined. Figure shows a parison of two transmission angles, A and B, drawn on the two radiation patterns. he isotropic antenna produces an omnidirectional radiation pattern of equal strength in all directions, so the A and B vectors are of equal length. For the antenna pattern of Figure , the B vector is longer than the A vector. indicating that more power is radiated in the B direction than in the A direction and the relative lengths of the two vectors are proportional to the amount of power radiated in the two directions. (a) Omnidirectional (b) DirectionalFigure Idealized Radiation Patterns The radiation pattern provides a convenient means of determining the beam width of an antenna, which is a mon measure of the directivity of an antenna. The beam width, also referred to as the halfpower beam width, is the angle within which the power radiated by the antenna is at least half of what it is in the most preferred direction.When an antenna is used for reception, he radiation pattern bees a reception pattern. The longest section of the pattern indicates the best direction for reception.Antenna TypesDipoles Two of the simplest and most basic antennas are the halfwave dipole, or Hertz. Antenna (Figure ) and the quarterwave vertical, or Marconi, antenna (Figure ).The halfwave dipole consists of two straight collinear conductors of equal length, separated by a small gap. The length of the antenna is onehalf the wavelength of the signal that can be transmitted most efficiently A vertical quarterwave antenna is the type monly used for automobile radios and portable radios.A halfwave dipole has a uniform or omnidirectional radiation pattern in one dimension and a figure eight pattern in the other two dimensions ().More plex antenna configurations can be used to produce a directional beam.(a) Halfwave dipole (b) Quarterwave antennaFigure Simple Antennas(a)Simple dipole(b)Directed antennaFigure Radiation Patterns in Three Dimensions [SCHl00]A typical directional radiation pattern is shown in Figure ：In this case the main strength of the antenna is in the xdirection.Parabolic Reflective Antenna An important type of antenna is the parabolic reflective antenna. Which is used in terrestrial microwave and satellite applications. A parabola is the locus of all points equidistant from a fixed 1ine and a fixed point not on the line. The parabola is revolved about its axis, the surface generated is called a paraboloid. A cross section through the paraboloid parallel to its axis forms a parabola and a cross section perpendicular to the axis forms a circle. Such surfaces are used in automobile headlights, optical and radio telescopes, and microwave antennas because of the following property：If a source of electromagnetic energy or sound)is placed at the focus of the paraboloid. and if the paraboloid is a reflecting surface, hen the wave will bounce back in lines parallel to the axis of the paraboloid；Figure shows this effect in cross section. In theory, this effect creates a parallel beam without dispersion. In practice, there will be some dispersion, because the source of energy must occupy more than one point. The converse is also true. If ining waves are parallel to the axis of the reflecting paraboloid the resulting signal will be concentrated at the focus.Figure shows a typical radiation pattern for the parabolic reflective antenna. and Table lists beam widths for antennas of various sizes at a frequency of 12GHz. Note that the larger the diameter of the antenna, he more tightly directional is the beam.Antenna GainAntenn