{天线与传播}.ppt

Antennas and Propagation(天线与传播) Chapter 5 Introduction An antenna is an electrical conductor　（电子导线） or system of conductors（导线系统） 发送 - radiates electromagnetic energy into space 接收 - collects electromagnetic energy from space In two-way communication, the same antenna can be used for transmission and reception 假定：两个方向上使用同一频率 Radiation Patterns（辐射模式） Radiation pattern Graphical representation of radiation properties of an antenna Depicted as two-dimensional cross section Beam width (or half-power beam width) （光束宽度或称半功率光束宽度） Measure of directivity of antenna （计算天线方向性的计量值） 半功率光束宽度表示一个角度，在这个角度内，天线辐射功率至少是该天线在最佳方向上辐射功率的一半 Reception pattern Receiving antenna’s equivalent to radiation pattern Types of Antennas Isotropic antenna (idealized) （各向同性天线） Radiates power equally in all directions 辐射模式：以天线为中心的一个球体 通常表示为三维模式的一个二维剖面 Dipole antennas （偶极天线） Half-wave dipole antenna (or Hertz antenna) （半波偶极天线或称赫兹天线） Quarter-wave vertical antenna (or Marconi antenna) （1/4波垂直天线或称马可尼天线） Parabolic Reflective Antenna （抛物反射天线） Antenna Gain （天线增益） Antenna gain Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) 天线增益不是为了似的输出功率大于输入功率，而主要是为了方向性 Effective area Related to physical size and shape of antenna 针对不同天线的经验计算公式见表5.2, 如：抛物线，抛物面面积为A，则其有效面积为0.56A. Antenna Gain Relationship between antenna gain and effective area G = antenna gain Ae = effective area f = carrier frequency c = speed of light (? 3 ′ 108 m/s) ? = carrier wavelength Propagation Modes Ground-wave propagation Sky-wave propagation Line-of-sight propagation Ground Wave Propagation Ground Wave Propagation Follows contour of the earth (沿地表传播) Can Propagate considerable distances Frequencies up to 2 MHz…??1km Example AM radio (调幅无线电) Sky Wave Propagation Sky Wave Propagation Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops, back and forth between ionosphere and earth’s surface R