【正文】 ther antennas, we must take into account the gain of the antenna, which yields the following free space loss equation：Where=gain of the transmitting antenna =gain of the receiving antenna=effective are a of the transmitting antenna=effective are a of the receiving antennaThe third fraction is derived from the second fraction using the relationship between antenna gain and effective area defined in Equation().We can recast this equation as： ()Thus, for the same antenna dimensions and separation, the longer the carrier wavelength (lower the carrier frequency f), the higher is the free space path loss. It is interesting to pare Equations () and (). Equation () indicates that as the frequency increases, the free space loss also increases, which would suggest that at higher frequencies, losses bee. more burdensome. However, Equation () shows that we can easily pensate for this increased loss with antenna gains. In fact, there is a net gain at higher frequencies, other factors remaining constant.Equation () shows that at a fixed distance an increase in frequency results in an increased loss measured by 20log(f). However, if we take into account antenna gain, and fix antenna area, then the change in loss is measured by 20log(f)。 atmospheric attenuation due to oxygen and water vaporExperimental。 radar。 quality varies with time of day, season, and frequencyAmateur radio。 slightly less reliable than VLF。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 parab