【正文】 oloid. 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 isotropic)Isotropic1Infinitesimal dipole or loopHalf –ware dipoleHorn ,mouth area A10A/Parabolic ,face area A7A/Turnstile (two crossed, perpendicular dipoles) PROPAGATION MODESA signal radiated from an antenna travels along one of three routes：ground wave, sky wave, or line of sight(LOS).Table shows in which frequency range each predominates. In this book, we are almost exclusively concerned with LOS Communication, but a short overview of each mode is given in this section.Ground Wave PropagationGround wave propagation (Figure ) more or less follows the contour of the earth and can propagate considerable distances well over the visual horizon. This effect is found in frequencies up to about 2 MHz. Several factors account for the tendency of electromagnetic wave in this frequency band to follow the earth’s curvature. One factor is that the electromagnetic wave induces a current in the earth’s surface, the result of which is to slow the wavefront near the earth, causing the wavefront to tilt downward and hence follow the earth’s curvature. Another factor is diffraction, which is a phenomenon having to do with the behavior of electromagnetic waves in the presence of obstacles.Electromagnetic waves in this frequency range are scattered by the atmosphere in such a way that they do not penetrate the upper atmosphere. The bestknown example of ground wave munication is AM radio.Sky Wave PropagationSky wave propagation is used for amateur radio, CB radio, and international broad casts such as BBC and Voice of America. With sky wave propagation a signal from an earthbased antenna is reflected from the ionized layer of the upper atmosphere (ionosphere) back down to earth. Although it appears the wave is reflected from the ionosphere as if the ionosphere were a hard reflecting surface the effect is in fact caused by refraction. Refraction is described subsequently.A sky wave signal can travel through a number of hops, bouncing back and forth between the ionosphere and the。 absorption in daytimeLongrange navigation。 international broadcasting, military munication。 microware links。 wireless local loop Infrared300GHz to400THz1mm to770nmLOSInfrared LANs。that is, there is actually a decrease in loss at higher frequencies.Figure Free Space LossNoiseFor any data transmission event, he received signal will consist of the transmitted signal, modified by the various distortions imposed by the transmission system, plus additional unwanted signals that are inserted somewhere between transmission and reception. These unwanted signals are referred to as noise. Noise is the major limiting factor in munications system performance.Noise may be divided into four categories： Intermodulation noiseAttenuation and attenuation distortion rainfall attenuation above 10GHz。 scattering because of temperature inversion。 and night SW。 Power line frequencies。 high atmospheric noise levelLongrange navigation。 direction finding。 cosmic noiseUHF television。 terrestrial microware links。Atmospheric absorption183