【正文】 s 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 GainAntenna Gain is a measure of the directionality of an antenna. Antenna gain is defined as the power output, in a particular direction, pared to that produce d in any direction by a perfect omnidirectional antenna (isotropic antenna). For example, (a)Parabola (b) Cross section of (c) Cross section of parabolic antenna parabolic antenna showing reflective property showing radiation patternFigure Parabolic Reflective AntennaTable Antenna Beam widths for Various Diameter Parabolic Reflective Antennas at f=12GHz [FREE97]Antenna Diameter (m)Beam width(degrees)If an antenna has a gain of 3 dB, that antenna improves upon the isotropic antenna in that direction by 3 dB, or a factor of increased power radiated in a given direction is at the expense of other directions. In effect increased power is radiated in one direction by reducing the power radiated in other directions. It is important to note that antenna gain does not refer to obtaining more output power than input power but rather to directionality. A concept related to that of antenna gain is the effective area of an antenna. The effective area of an antenna is related to the physical size of the antenna and to its shape. The relationship between antenna gain and effective area is ()WhereG=antenna gain=effective areaf=carrier frequencyC=speed of light(≈3m/s)λ=carrier wavelengthTable shows the antenna gain and effective area of some typical antenna shapes.Table Antenna Gains and Effective Areas [COUC01]Type of AntennaEffective Area Power Gain (relative to is