【正文】 a 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。 quality varies with time of day, season, and frequencyAmateur radio。 atmospheric attenuation due to oxygen and water vaporExperimental。Crosstalk atmospheric attenuation due to oxygen and water vaporSatellite munication。 attenuation low at night, high in day。 low attenuation day and night。 FM broadcast and twoway radio, AM aircraft navigation aidsBandFrequencyFreeSpace WavelengthRangePropagation CharacteristicsTypical UseUHF(ultra high frequency)300MHz to3000MHz100cm to10cmLOS。NoiseMultipath. cellular telephone。 submarine municationLF(low frequency)30kHz to300kHz10km to1kmGW。 marine munication radio beaconsMF(medium frequency)300kHz to3000kHz1000m to100mGW。 personal munications systemSHF(super high frequency)3GHz to30GHz10cm to1cmLOS。Tnermal noise consumer electronic applicationsVisible light400THz to900THz770nm to330nmLOSOptical municationGround wave propagationSky wave propagation (2 to 30 MHz)Lineofsight (LOS) propagation(above 30 MHz) Wireless Propagation ModesFigure Refraction of an Electromagnetic Wave [POOL98]Optical and Radio Line of Sight With no intervening obstacles, the optical line of sight can be expressed asWhere d is the distance between an antenna and the horizon in kilometers and h is the antenna height in meters. The effective, or radio, line of sight to the horizon is expressed as(Figure )：Figure Optical and Radio HorizonsWhere K is an adjustment factor to account for the refraction. A good rule of thumb is K=4/,the maximum distance between two antennas for LOS propagation is ,where and are the heights of the two antennas. LINEOFSIGHT TRANSMISSIONWith any munications system, the signal that is received will differ from the signal that is transmitted, due. o various transmission impairments. For analog signals, these impairments introduce various random modifications that degrade the signal quality. For digital errors are introduced：A binary I is transformed into a binary 0,and vice versa. In this section we examine the various impairments and ment on their effect on the informationcarrying capacity of