【正文】 otropic)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。 earth’s surface (Figure ).with this propagation mode, a signal can be picked up thousands of kilometers from the transmitter.LineofSight PropagationAbove 30 MHz, neither ground wave nor sky wave propagation modes operate, and munication must be by line of sight (Figure ).For satellite munication, a signal above 30 MHz is not reflected by the ionosphere and therefore can be transmitted between an earth station and a satellite overhead that is not beyond the horizon. For ground—based munication, the transmitting and receiving antennas must be within an effective line of sight of each other. The term effective is used because microwaves are bent or refracted by the atmosphere. The amount and even the direction of the bend depends on conditions, but generally microwaves are bent with the curvature of the earth and will therefore propagate farther than the optical line of sight.Refraction Before proceeding, a brief discussion of refraction is warranted. Refraction occurs because the velocity of an electromagnetic wave is a function of the density of the medium through which it travels In a vacuum, an electromagnetic wave (such as light or a radio wave) travels at approximately 3m/s. This is the constant, c, monly referred to as the speed of light, but actually referring to the speed of light in a vacuum. In air, water, glass, and other transparent or partially transparent media, electromagnetic waves travel at speeds less than c.When an electromagnetic wave moves from a medium of one density to a medium of another density, its speed changes. The effect is to cause a onetime bending of the direction of the wave at the boundary between the two media. This is illustrated in moving from a less dense to a more dense medium, he wave will bend toward the more dense medium. This phenomenon IS easily observed by partially immersing a stick in water. The result will look much like Figure with the stick appearing shorter and bent.The index of refraction of one medium relative to another is the sine of the angle of incidence divided by the sine of the angle of refraction. The index of refraction is also equal to the ratio of the respective velocities in the two media. The absolute index of refraction of a medium is calculated in parison with that of a vacuum. Refractive index varies with wavelength, o that refractive effects differ for signals with different wavelengths.Although Figure shows an abrupt, onetime change in direction as a signal moves from one medium to another, a continuous, gradual bending of a signal will occur if it is moving through a medium in which the index of refraction gradually changes. Under normal propagation conditions, the refractive index of the atmosphere decreases with height so that radio waves travel more slowly near the ground than at higher altitudes. The result is a slight bending of the radio waves toward the earth.Table Frequency BandsBandFrequencyFreeSpace WavelengthRangePropagation CharacteristicsTypical UseELF(extremely low frequency)30Hz to300Hz10000km to1000kmGW。 used by some home control systemsVF(voice frequency)300Hz to3000Hz1000km to100kmGWUsed by the telephone system for analog subscriber linesVLF(very low frequency)3kHz to30kHz100km to10kmGW。 high atmospheric noise levelLongrange navigation。 slightly less reliable than VLF。 marine munication radio beaconsMF(medium frequency)300kHz to3000kHz1000m to100mGW。 attenuation low at night, high in day。 direction finding。 quality varies with time of day, season, and frequencyAmateur radio。 Longdistance aircraft and ship municationVHF(very high frequency)30MHz to300MHz10m to1mLOS。 cosmic noiseVHF television。 cosmic noiseUHF television。 radar。 personal munications systemSHF(super high frequency)3GHz to30GHz10cm to1cmLOS。 atmospheric attenuation due to oxygen and water va