信息科学与电子工程专业英语Lecture03 n课件.ppt

1,任爱锋电子工程学院 E-mail: Office: 新科技楼A901 Phone: 88202830-607 Mobile: 13384919599,2,Unit 3 电磁场, 天线和微波EM Fields, Antenna and Microwaves,Unit 3-1 电磁场,Electromagnetic Field,3,New Words,4,The electromagnetic field is a physical field produced by electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction. It is one of the four fundamental forces in the nature (the others are gravitation, the weak interaction, and the strong interaction),电磁场是由带电物体产生的物理场。它会影响场附近的带电物体的行为。电磁场在空间无限延伸，描述电磁相互作用。它是自然界中四个基本作用力之一（其余为万有引力，弱相互作用，强相互作用）。,Unit 3-1 电磁场,5,The field can be viewed as the combination of an electric field and a magnetic field. The electric field is produced by stationary charges, and the magnetic field by moving charges (currents); these two are often described as the sources of the field. The way in which charges and currents interact with the electromagnetic field is described by Maxwells equations and the Lorentz force law.,电磁场可以看成是电场和磁场的结合。电场是由静止电荷产生的，磁场由运动电荷（电流）产生；这两种（电荷）通常被描述为电磁场的源。电荷和电流与电磁场相互作用的方式由麦克斯韦方程组和洛伦兹力定律所描述。,Unit 3-1 电磁场,6,From a classical point of view, the electromagnetic field can be regarded as a smooth, continuous field, propagated in a wavelike manner, whereas from a quantum mechanical point of view, the field can be viewed as being composed of photons.,从经典的角度，电磁场可认为是平滑、连续的场，以波动方式传播；而从量子力学的角度，场可看作是由光子组成的。,Unit 3-1 电磁场,7,The electromagnetic field may be viewed in two distinct ways.Continuous structure: Classically, electric and magnetic fields are thought of as being produced by smooth motions of charged objects. For example, oscillating charges produce electric and magnetic fields that may be viewed in a “smooth”, continuous, wavelike manner.,电磁场可用两种截然不同的观点来看。连续结构：经典地，电场和磁场被认为是由带电物体的平滑运动产生的。例如，振荡电荷产生电场和磁场，可看成光滑的、连续的波动方式。,Unit 3-1 电磁场电磁场的结构,8,In this case, energy is viewed as being transferred continuously through the electromagnetic field between any two locations. For instance, the metal atoms in a radio transmitter appear to transfer energy continuously. This view is useful to a certain extent (radiation of low frequency), but problems are found at high frequencies (see ultraviolet catastrophe). This problem leads to another view.,这时，能量可以看成是在任意两个位置之间通过电磁场连续地传递。例如，在无线电发射机中的金属原子（看来是）连续地传递能量。这个观点在一定程度上（低频辐射）是有用的，但是高频时就有问题（如紫外灾难）。由此产生了另一种观点。,Unit 3-1 电磁场电磁场的结构,The ultraviolet catastrophe, also called the Rayleigh-Jeans catastrophe, was a prediction of early 20th century classical physics that an ideal black body at thermal equilibrium will emit radiation with infinite power.,9,Discrete structure: The electromagnetic field may be thought of in a more “coarse” way. Experiments reveal that electromagnetic energy transfer is better described as being carried away in photons with a fixed frequency. Plancks relation links the energy E of a photon to its frequency n through the equation: E = h n,离散结构：电磁场可以一种比较“粗略”的方式来考虑。实验表明：电磁场的能量传递可以更好地描述为用固定频率的光子来传递。普朗克的关系式将光子的能量E及其频率v 通过下式联系起来：E = h n,Unit 3-1 电磁场电磁场的结构,10,where h is Plancks constant, named in honor of Max Planck, and n is the frequency of the photon. For example, in the photoelectric effect the emission of electrons from metallic surfaces by electromagnetic radiation it is found that increasing the intensity of the incident radiation has no effect, and that only the frequency of the radiation is relevant in ejecting electrons.1,其中h是为纪念马克斯普朗克而命名的普朗克常量。v是光子的频率。例如在光电效应中，即因电磁辐射而从金属表面发射电子的现象，我们发现增加入射辐射的强度并无影响，只有辐射频率与发射的电子有关。,Unit 3-1 电磁场电磁场的结构,11,This quantum picture of the electromagnetic field has proved very successful, giving rise to quantum electrodynamics, a quantum field theory describing the interaction of electromagnetic radiation with charged matter.,已经证明电磁场的量子描述是非常成功的，引出了量子电动力学。量子电动力学是一种描述电磁辐射与带电物体之间相互作用的量子场理论。,Unit 3-1 电磁场电磁场的结构,12,In the past, electrically charged objects were thought to produce two types of field associated with their charge property. An electric field is produced when the charge is stationary with respect to an observer measuring the properties of the charge and a magnetic field (as well as an electric field) is produced when the charge moves (creating an electric current) with respect to this observer.,过去，认为带电物体会产生与它们电荷性质有关的两种场。相对于测量电荷性质的观察者，电荷静止时产生电场，电荷运动（产生电流）时产生磁场（和电场）。,Unit 3-1 电磁场电磁场的动力学(Dynamics of the electromagnetic field),13,Over time, it was realized that the electric and magnetic fields are better thought of as two parts of a greater whole the electromagnetic field.2,随着时间的推移，人们认识到电场和磁场是电磁场这一整体的两个部分。,Unit 3-1 电磁场电磁场的动力学(Dynamics of the electromagnetic field),14,Once this electromagnetic field has been produced from a given charge distribution, other charged objects in this field will experience a force (in a similar way that planets experience a force in the gravitational field of the Sun). If these other charges and currents are comparable in size to the sources producing the above electromagnetic field, then a new net electromagnetic field will be produced.3,一旦给定的电荷分布产生了电磁场，那么在电磁场中的其它带电物体就会受到一个力的作用（就像行星受到太阳重力场的力一样）。如果这些电荷和电流的大小与产生上述电磁场的源是可比的，那么将产生一个新的净电磁场。,Unit 3-1 电磁场电磁场的动力学(Dynamics of the electromagnetic field),15,Thus, the electromagnetic field may be viewed as a dynamic entity that causes other charges and currents to move, and which is also affected by them. These interactions are described by Maxwells equations and the Lorentz force law.,这样，电磁场可以认为是一个动力学实体，它促使其它电荷和电流运动，同时又受它们影响。这些相互作用可由麦克斯韦方程和洛伦兹力定律来描述。,Unit 3-1 电磁场电磁场的动力学(Dynamics of the electromagnetic field),16,Unit 3 电磁场, 天线和微波EM Fields, Antenna and Microwaves,Unit 3-2 微带天线,Microstrip Antenna,17,New Words,18,New Words,19,In telecommunication, there are several types of microstrip antennas (also known as printed antennas) the most common of which is the microstrip patch antenna or patch antenna. A patch antenna is a narrowband, wide-beam antenna fabricated by etching the antenna element pattern in metal trace bonded to an insulating dielectric substrate with a continuous metal layer bonded to the opposite side of the substrate which forms a ground plane.1,通信中，有几种微带天线（又叫印刷天线），其中最普通的是微带片状天线 或片状天线。片状天线是一种窄带宽波束的天线，通过将天线单元图样蚀刻到粘贴在绝缘基底上的金属轨迹而制成，基底的另一面则粘贴连续的金属层形成接地平面。,Unit 3-2 微带天线,20,Common microstrip antenna radiator shapes are square, rectangular, circular and elliptical, but any continuous shape is possible. Some patch antennas eschew a dielectric substrate and suspend a metal patch in air above a ground plane using dielectric spacers; the resulting structure is less robust but provides better bandwidth.,通常微带天线辐射器的形状有正方形的，矩形的，圆形的和椭圆的，而任何连续的形状都是可以。一些片状天线为了避免绝缘基底，在接地平面的上空用绝缘逆电流器悬挂一个金属贴片；这种结构的鲁棒性不是很好，但能提供更好的带宽。,Unit 3-2 微带天线,21,Because such antennas have a very low profile, are mechanically rugged and can be conformable, they are often mounted on the exterior of aircraft and spacecraft, or are incorporated into mobile radio communications devices.2,因为这种天线外形低矮，机械强度大，并且形状上适应性强，通常装在飞机或太空飞行器外表，或组合在移动无线电通信设备上。,Unit 3-2 微带天线,22,Microstrip antennas are also relatively inexpensive to manufacture and design because of the simple 2-dimensional physical geometry. They are usually employed at UHF and higher frequencies because the size of the antenna is directly tied to the wavelength at the resonant frequency. A single patch antenna provides a maximum directive gain of around 6-9 dBi.,因为其简单的二维形状，所以微带天线的生产和设计相对来说不算太贵。微带天线通常用于UHF（超高频）和更高的频率，因为天线的尺寸直接受制于谐振频率的波长。一个简单的片状天线可以提供最大为69dBi的定向增益。,Unit 3-2 微带天线,dBi: decibel isotropic, the forward gain of an antenna compared with the isotropic antenna, which uniformly distributes energy in all directions.,23,It is relatively easy to print an array of patches on a single (large) substrate using lithographic techniques. Patch arrays can provide much higher gains than a single patch at little additional cost; matching and phase adjustment can be performed with printed microstrip feed structures, again in the same operations that form the radiating patches.,通过平板印刷技术我们很容易在单个大绝缘体上印刷贴片阵列。以很少的额外成本，贴片阵列比单个贴片能提供更高的增益，用形成辐射电波的片状微带天线的同样机理，可对印刷微带天线的馈电结构进行匹配和相位调节。,Unit 3-2 微带天线,24,The ability to create high gain arrays in a low-profile antenna is one reason that patch arrays are common on airplanes and in other military applications.,以低矮的天线构成高增益阵列的能力是片状天线广泛用于飞机和其他军事应用的原因之一。,Unit 3-2 微带天线,25,The most commonly employed microstrip antenna is a rectangular patch. The rectangular patch antenna is approximately a one-half wavelength long section of rectangular microstrip transmission line. When air is the antenna substrate, the length of the rectangular microstrip antenna is approximately one-half of a free-space wavelength.,最普遍应用的微带天线是矩形贴片。矩形贴片天线长近似为矩形微带传输线的一段，其长度等于半波长。当空气作为天线基底时，矩形微带天线的长度大约为自由空间波长的一半。,Unit 3-2 微带天线,26,Since the antenna is loaded with a dielectric as its substrate, the length of the antenna decreases as the relative dielectric constant of the substrate increases. The resonant length of the antenna is slightly shorter because of the extended electric “fringing fields” which increase the electrical length of the antenna slightly.,因为天线是负载绝缘体作为它的基底，所以天线的长度会随着基底的相对介电常数的增加而减小。天线的谐振长度略短一些，这是因为“边缘场”稍微增加了天线的电气长度。,Unit 3-2 微带天线,Fringing fields: the electric fields produced by scattered electrons in an electron microscope.,27,An early model of the microstrip antenna is a section of microstrip transmission line with equivalent loads on either end to represent the radiation loss.,微带天线的一个早期模型是一段微带传输线，在一端具有等效负载，代表辐射损耗。,Unit 3-2 微带天线,28,The dielectric loading of a microstrip antenna affects both its radiation pattern and impedance bandwidth. As the dielectric constant of the substrate increases, the antenna bandwidth decreases which increases the Q factor of the antenna and therefore decreases the impedance bandwidth.3,微带天线的介电负载影响天线的辐射模式和阻抗带宽。随着基底介电常数的增大，天线的带宽减小，其Q值提高，因而阻抗带宽减小。,Unit 3-2 微带天线,29,This relationship did not immediately follow when using the transmission line model of the antenna, but is apparent when using the cavity model which was introduced in the late 1970s. The radiation from a rectangular microstrip antenna may be understood as a pair of equivalent slots.,当使用天线的传输线模型时这种情况并没有发生，到了七十年代后期出现空腔模型时这种情况就显现出来了。,Unit 3-2 微带天线,30,These slots act as an array and have the highest directivity when the antenna has an air dielectric and decreases as the antenna is loaded by material with increasing relative dielectric constant.,矩形微带天线的辐射可以理解为一对等效的狭缝。这些狭缝就像一个阵列，当天线使用空气电介质时方向性最好，但是当天线的负载材料的相对介电常数增加时方向性会减弱。,Unit 3-2 微带天线,31,An advantage inherent to patch antennas is the ability to have polarization diversity. Patch antennas can easily be designed to have various polarizations, using multiple feed points, or a single feed point with asymmetric patch structures.4 This unique property allows patch antennas to be used in many types of communications links that may have varied requirements.,贴片天线固有的优点是能有极化分集。片状天线能很容易地通过多个馈点或在非对称片结构上采用单一馈点设计成具有各种极化特性。这个独特的性质使得贴片天线应用于有不同需求的通信链接的许多领域。,Unit 3-2 微带天线,32,The half-wave rectangular microstrip antenna has a virtual shorting plane along its center. This may be replaced with a physical shoring plane to create a quarter-wavelength microstrip antenna. This is sometimes called a half-patch.,半波矩形微带天线沿其中心有一个虚拟短路平面。用物理短路平面替代上述虚拟短路平面就可以做成四分之一波长微带天线。有时称四分之一波长微带天线为半贴片。,Unit 3-2 微带天线,33,The antenna only has a single radiation edge (equivalent slot) which lowers the directivity/gain of the antenna. The impedance bandwidth is slightly lower than a half-wavelength full patch as the coupling between radiating edges has been eliminated.,这种天线只有单一的辐射边缘（等效狭缝），因此会降低天线的方向性/增益。由于没有辐射边缘之间的耦合，所以阻抗带宽也比半波长全贴片窄。,Unit 3-2 微带天线,天 线 与电 波 传 播,天线1.1 天线的作用与地位 无线电发射机输出的射频信号功率，通过馈线（电缆）输送到天线，由天线以电磁波形式辐射出去。电磁波到达接收地点后，由天线接下来（仅仅接收很小很小一部分功率），并通过馈线送到无线电接收机。可见，天线是发射和接收电磁波的一个重要的无线电设备，没有天线也就没有无线电通信。 天线品种繁多，以供不同频率、不同用途、不同场合、不同要求等不同情况下使用。 对于众多品种的天线，进行适当的分类是必要的： 按用途分类 可分为通信天线、电视天线、雷达天线等； 按工作频段分类 可分为短波天线、超短波天线、微波天线等； 按方向性分类 可分为全向天线、定向天线等； 按外形分类 可分为线状天线、面状天线等.,天线的基本知识,* 电磁波的辐射 导线上有交变电流流动时，就可以发生电磁波的辐射，辐射的能力与导线的长度和形状有关。如 图1.1 a 所示，若两导线的距离很近，电场被束缚在两导线之间，因而辐射很微弱；将两导线张开，如 图1.1 b 所示，电场就散播在周围空间，因而辐射增强。 必须指出，当导线的长度 L 远小于波长 时，辐射很微弱；导线的长度 L 增大到可与波长相比拟时，导线上的电流将大大增加，因而就能形成较强的辐射。,天线的基本知识,图1.1 a,图1.1 b,1.2 对称振子 对称振子是一种经典的、迄今为止使用最广泛的天线，单个半波对称振子可简单地单独立地使用或用作为抛物面天线的馈源，也可采用多个半波对称振子组成天线阵。 两臂长度相等的振子叫做对称振子。每臂长度为四分之一波长、全长为二分之一波长的振子，称半波对称振子, 见 图1.2 a 。 另外，还有一种异型半波对称振子，可看成是将全波对称振子折合成一个窄长的矩形框，并把全波对称振子的两个端点相叠，这个窄长的矩形框称为折合振子，注意，折合振子的长度也是为二分之一波长，故称为半波折合振子, 见 图1.2 b 。,天线的基本知识,图1.2 a,图1.2 b,1.3 天线方向性的讨论1.3.1 天线方向性 发射天线的基本功能之一是把从馈线取得的能量向周围空间辐射出去，基本功能之二是把大部分能量朝所需的方向辐射。 垂直放置的半波对称振子具有平放的 “面包圈” 形的立体方向图（图1.3.1 a）。 立体方向图虽然立体感强，但绘制困难， 图1.3.1 b 与图1.3.1 c 给出了它的两个主平面方向图，平面方向图描述天线在某指定平面上的方向性。从图1.3.1 b 可以看出，在振子的轴线方向上辐射为零，最大辐射方向在水平面上；而从图1.3.1 c 可以看出，在水平面上各个方向上的辐射一样大。,天线的基本知识,图1.3.1 c 水平面方向图,图1.3.1 b 垂直面方向图,图1.3.1 a 立体方向图,天线的基本知识,1.3.2 天线方向性增强 若干个对称振子组阵，能够控制辐射，产生“扁平的面包圈” ，把信号进一步集中到在水平面方向上。 下图是4个半波对称振子沿垂线上下排列成一个垂直四元阵时的立体方向图和垂直面方向图。,垂直面方向图,立体方向图,也可以利用反射板可把辐射能控制到单侧方向 平面反射板放在阵列的一边构成扇形区覆盖天线。下面的水平面方向图说明了反射面的作用-反射面把功率反射到单侧方向，提高了增益。,天线的基本知识,全向阵（垂直阵列 不带平面反射板）,抛物反射面的使用，更能使天线的辐射，像光学中的探照灯那样，把能量集中到一个小立体角内，从而获得很高的增益。不言而喻，抛物面天线的构成包括两个基本要素：抛物反射面 和 放置在抛物面焦点上的辐射源。,扇形区覆盖（垂直阵列 带平面反射板）,平面反射板,天线的基本知识,1.3.3 增益 增益是指：在输入功率相等的条件下，实际天线与理想的辐射单元在空间同一点处所产生的信号的功率密度之比。它定量地描述一个天线把输入功率集中辐射的程度。增益显然与天线方向图有密切的关系，方向图主瓣越窄，副瓣越小，增益越高。 可以这样来理解增益的物理含义-为在一定的距离上的某点处产生一定大小的信号，如果用理想的无方向性点源作为发射天线，需要100W的输入功率，而用增益为 G = 13 dB = 20 的某定向天线作为发射天线时，输入功率只需 100 / 20 = 5W . 换言之，某天线的增益，就其最大辐射方向上的辐射效果来说，与无方向性的理想点源相比，把输入功率放大的倍数。 半波对称振子的增益为G = 2.15 dBi ; 4个半波对称振子 沿垂线上下排列，构成一个垂直四元阵，其增益约为G = 8.15 dBi ( dBi这个单位表示比较对象是各向均匀辐射的理想点源) 。 如果以半波对称振子作比较对象，则增益的单位是dBd . 半波对称振子的增益为G = 0 dBd （因为是自己跟自己比，比值为1，取对数得零值。） ； 垂直四元阵，其增益约为G = 8.15 2.15 = 6 dBd .,1.3.4 波瓣宽度 方向图通常都有两个或多个瓣，其中辐射强度最大的瓣称为主瓣，其余的瓣称为副瓣或旁瓣。参见图1.3.4 a , 在主瓣最大辐射方向两侧，辐射强度降低 3 dB（功率密度降低一半）的两点间的夹角定义为波瓣宽度（又称 波束宽度 或 主瓣宽度 或 半功率角）。波瓣宽度越窄，方向性越好，作用距离越远，抗干扰能力越强。 还有一种波瓣宽度，即 10dB波瓣宽度，顾名思义它是方向图中辐射强度降低 10dB （功率密度降至十分之一） 的两个点间的夹角，见图1.3.4 b .,天线的基本知识,3dB 波瓣宽度,- 3dB点,- 3dB点,10dB 波瓣宽度,-10dB点,- 10dB点,峰值方向（最大辐射方向）,图1.3.4 a,图1.3.4 b,峰值方向（最大辐射方向）,天线的基本知识,1.3.5 前后比 方向图中，前后瓣最大值之比称为前后比，记为 F / B 。前后比越大，天线的后向辐射（或接收）越小。前后比F / B 的计算十分简单- F / B = 10 Lg （前向功率密度） /（ 后向功率密度 ） 对天线的前后比F / B 有要求时，其典型值为 （18 30）dB，特殊情况下则要求达（35 40）dB .,1.3.6 天线增益的若干近似计算式 1）天线主瓣宽度越窄，增益越高。对于一般天线，可用下式估算其增益： G（ dBi ） = 10 Lg 32000 / （ 23dB,E 23dB,H ） 式中， 23dB,E 与 23dB,H 分别为天线在两个主平面上的波瓣宽度； 32000 是统计出来的经验数据。 2）对于抛物面天线，可用下式近似计算其增益： G（ dB i ） = 10 Lg 4.5 （ D / 0 ）2 式中， D 为抛物面直径； 0 为中心工作波长； 4.5 是统计出来的经验数据。 3）对于直立全向天线，有近似计算式 G（ dBi ） = 10 Lg 2 L / 0 式中， L 为天线长度； 0 为中心工作波长；,天线的基本知识,1.3.7 上旁瓣抑制 对于基站天线，人们常常要求它的垂直面（即俯仰面）方向图中，主瓣上方第一旁瓣尽可能弱一些。这就是所谓的上旁瓣抑制 。基站的服务对象是地面上的移动电话用户，指向天空的辐射是毫无意义的。,天线的基本知识,1.3.8 天线的下倾 为使主波瓣指向地面，安置时需要将天线适度下倾。,天线的基本知识,垂直极化,1.4 天线的极化 天线向周围空间辐射电磁波。电磁波由电场和磁场构成。人们规定：电场的方向就是天线极化方向。一般使用的天线为单极化的。下图示出了两种基本的单极化的情况：垂直极化-是最常用的；水平极化-也是要被用到的。,天线的基本知识,水平极化,垂直极化,1.4.1 双极化天线 下图示出了另两种单极化的情况：+45 极化 与 -45 极化，它们仅仅在特殊场合下使用。这样，共有四种单极化了，见下图。 把垂直极化和水平极化两种极化的天线组合在一起，或者，把 +45 极化和 -45 极化两种极化的天线组合在一起，就构成了一种新的天线-双极化天线。,天线的基本知识,水平极化,+45 极化,-45 极化,下图示出了两个