复习课--天线与电波传播(精华)

第4章口径（面）天线4.1惠更斯元的辐射面天线的结构包括金属导体面S′、金属导体面的开口径S（即口径面）及由S0=S′+S所构成的封闭曲面内的辐射源,如图4-1 所示。

由于在封闭面上有一部分是导体面S′, 所以其上的场为零, 这样使得面天线的辐射问题简化为口径面S的辐射，即S0=S′+S→S, 设口径上的场分布E S, 根据惠更斯-菲涅尔原理,, 称为惠更斯元。

把口径面分割为许多面元dS●源SS′图4-1 面天线的原理由面元上的场分布即可求出其相应的辐射场, 然后再在整个口径面上积分便可求出整个口径的辐射场。

下面先来分析惠更斯元的辐射场。

　如同电基本振子和磁基本振子是分析线天线的基本辐射单元一样, 惠更斯元是分析面天线的基本辐射单元。

设平面口径上一个惠更斯元d S =d x d y , 若面元上的切向电场为E y , 切向磁场为H x , 则根据等效原理, 面元上的磁场等效为沿y 轴方向放置, 电流大小为H x d x 的电基本振子; 而面元上的电场则等效为沿x 轴方向放置, 磁流大小为E y d y 的磁基本振子。

因而惠更斯元可视为两正交的长度为d y 、大小为H x d x 的电基本振子与长度为d x 、大小为E y d y 的磁基本振子的组合, 如图4 -2 所示, 其中为惠更斯元d S 的外法线矢量。

它的电流矩和磁流矩分别为z ρ图4 –2 惠更斯元zrd S yxnOd y d xH x E yI y l=(H x d x )d y =H x d S I M x l=(E y d y )d x =E y d S类似第1章沿z 轴放置的电基本振子的辐射场，可得沿y 轴放置的电基本振子辐射场为:[]ϕϑληϑθcos sin cos 2a a e r l I j E jkry +−=−[]ϕϕθληϑϕcos sin cos 2a a e rl I j H jkry −−=−同样可得沿x 轴放置的磁基本振子的远区场表达式:[]ϑθϑλϑθcos cos sin 2a a e rl I j E jkrM x+=−（4-1-1）（4-1-2）（4-1-3）[]ϑϑθηλϑθsin sin cos 2a a e rl I j H jkrM x−−=−将式（4 -1 -1）代入上两式, 可得惠更斯元的辐射场为⎥⎦⎤⎢⎣⎡+++=−)cos 1(cos )cos (sin 2θηϕθηϕλθθx y x y jkry H E a H E a erds E jdE 对于平面波, 有E y /H x =η, 因此上式简化为)cos 1(2θλ+=−jkry erds E jdE 在研究天线方向性时, 通常是关心两个主平面的情况, 所以,我们只介绍面元的两个主平面的辐射。

天线与电波传播天线部分:引言天线是一种用来发射或接收电磁波的器件，是任何无线电系统中的基本组成部分。

换句话说，发射天线将传输线中的导行电磁波转换为“自由空间”波，接收天线则与此相反。

于是信息可以在不同地点之间不通过任何连接设备传输，可用来传输信息的电磁波频率构成了电磁波谱。

人类最大的自然资源之一就是电磁波谱，而天线在利用这种资源的过程中发挥了重要的作用。

第一讲:传输线基础知识在通信系统中，传输线(馈线)是连接发射机与发射天线或接收机与接收天线的器件。

为了更好的了解天线的性能及参数，首先简单介绍有关传输线的基础知识。

传输线根据频率的使用范围区分有两种类型:1、低频传输线;2、微波传输线。

这里重点介绍微波传输线中无耗传输线的基础知识，主要包括反映传输线任Z一点特性的参量:反射系数、阻抗和驻波比,。

,一、反射系数 ,这里定义传输线上任一点处的电压反射系数为,'Uz()',,()z，'Uz()'jz,,,',Uze0，， (1) ,'jz，',,Uze0，，'jz,2,,,el由上式可以看出，反射系数的模是无耗传输线系统的不变量，即',,,z (2) ，，l此外，反射系数呈周期性，即'',，,,zmz,/2 (3) ，，，，gZ二、阻抗appearance of the weld appearance quality technical requirements of the project must not have a molten metal stream does not melt the base metal to weld, weld seam and heat-affected zone surface must not have cracks, pores, defects such as crater and ash, surface smoothing, weld and base metal should be evenly smooth transition. Width 2-3 mm from the edge of weld Groove. Surface reinforcement should be less than or equal to 1 + 0.2 times the slope edge width, and should not be greater than 4 mm. Depth of undercut should be less than or equal to 0.5 mm, total length of the welds on both sides undercut not exceed 10% of the weld length, and long continuous should not be greater than 100 mm. Wrongside should be less than or at 0.2T, and should not be greater than 2 mm (wall thickness mm t) incomplete or not allow 7.5 7.5.1 installation quality process standards of the electrical enclosure Cabinet surface is clean, neat, no significant phenomenon of convex, close to nature, close the door. 7.5.2 Cabinet Cabinet face paints no paint, returned to rusted, consistent color. 7.5.3 uniform indirect gap from top to bottom, slot width <1.5mm 7.5.4 adjacent Cabinet surface roughness is 0. 7.5.5 the cabinets firmly fixed, crafts beautiful. 7.5.6 Cabinet surface gauge, switch cabinet mark clear, neat, firm paste. 7.5.7 Terminal row of neat, is reliable, the appearance is clean and not damaged. 7.5.8 cables neat and clean, solid binding, binding process in appearance. 7.5.9 the first cable production firm, crafts beautiful, clear signage does not fade.7.5.10 fireproof plugging tight, no cracks and pores. 7.6 7.6.1 of thestandard electrical wiring quality technology cable a, the multi-core wire bunch arrangement should be parallel to each other, horizontal wire harness or wire should be perpendicular to the longitudinal multi-core wire bunch. The distance between the wire harness and wire harness symmetry, and as close as possible. B-core wiring harness into round, multi-core wire bunch used g wire binding, fastening这里定义传输线上任一点处的阻抗为'Uz，，' (4) Zz,，，'Iz，，经过一系列的推导，得出阻抗的最终表达式'ZjZz，tan,'l0 (5) ZzZ,，，0'ZjZz，tan,l0三、驻波比(VSWR) ,这里定义传输线上任一点处的驻波比为'Uz，，max,, (6) 'Uz，，min经过一系列的推导，得出阻抗的最终表达式1，,l (7) ,,1,,l此外，这里还给出反射系数与阻抗的关系表达式'1，,z，，'ZzZ,，，0'1,,z，，(8) 'ZzZ,，，0',,z，，'ZzZ，，，0这里还简单介绍一下传输线理论所要用到的一些基本参数，例如特性阻抗Z0 以及相位常数，具体表达式如下: ,L2,,,,ZLC, (9) ,,0C,Z此外，不同的系统有不同的特性阻抗，为了统一和便于研究，常常提出归一化0'Zz，，的概念，即阻抗称为归一化阻抗 Z0'Zz，，'Zz, (10) ，，Z0第二讲:基本振子的辐射appearance of the weld appearance quality technical requirements of the project must not have a molten metal stream does not melt the base metal to weld, weld seam and heat-affected zone surface must not have cracks, pores, defects such as crater and ash, surface smoothing, weld and base metal should be evenly smooth transition. Width 2-3 mm from the edge of weld Groove. Surface reinforcement should be less than or equal to 1 + 0.2 times the slope edge width, and should not be greater than 4 mm. Depth of undercut should be less than or equal to 0.5 mm, total length of the welds on both sides undercut not exceed 10% of the weld length, and long continuous should not be greater than 100 mm. Wrongside should be less than or at 0.2T, and should not be greater than 2 mm (wall thickness mm t) incomplete or not allow 7.5 7.5.1 installation quality process standards of the electrical enclosure Cabinet surface is clean, neat, no significant phenomenon of convex, close to nature, close the door. 7.5.2 Cabinet Cabinet face paints no paint, returned to rusted, consistent color. 7.5.3 uniform indirect gap from top to bottom, slot width <1.5mm 7.5.4 adjacent Cabinet surface roughness is 0. 7.5.5 the cabinets firmly fixed, crafts beautiful. 7.5.6 Cabinet surface gauge, switch cabinet mark clear, neat, firm paste. 7.5.7 Terminal row of neat, is reliable, the appearance is clean and not damaged. 7.5.8 cables neat and clean, solid binding, binding process in appearance. 7.5.9 the first cable production firm, crafts beautiful, clear signage does not fade.7.5.10 fireproof plugging tight, no cracks and pores. 7.6 7.6.1 of the standard electrical wiring quality technology cable a, the multi-core wire bunch arrangement should be parallel to each other, horizontal wire harness or wire should be perpendicular to the longitudinal multi-core wire bunch. The distance between the wire harness and wire harness symmetry, and as close as possible. B-core wiring harness into round, multi-core wire bunch used g wire binding, fastening一、电基本振子的辐射电基本振子(Electric short Dipole)又称电流元，无穷小振子或赫兹电偶极子，l,l它是指一段理想的高频电流直导线，其长度远小于波长，其半径远小于，a 同时振子沿线的电流处处等幅同相。