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1、Technical English,For Information Science and Electronic Engineering,Unit 3,EM Fields, Antenna and Microwaves,Part I,Electromagnetic Field,3,New Words,4,1,The electromagnetic field is a physical field produced by electrically charged objects. It affects the behavior of charged objects in the vicinit
2、y 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),电磁场在空间无限延伸，并描述电磁相互作用。,5,2,The field can
3、 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 elect
4、romagnetic field is described by Maxwells equations and the Lorentz force law.,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。,6,3,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
5、view, the field can be viewed as being composed of photons.,从经典的角度，电磁场可认为是光滑、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。,7,4 Structure of the electromagnetic field,The electromagnetic field may be viewed in two distinct ways. Continuous structure: Classically, electric and magnetic fields are thought of as
6、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.,例如，振荡电荷产生电场和磁场，可看成平滑的、连续的波动方式。,8,5,In this case, energy is viewed as being transferred continuously through the el
7、ectromagnetic 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 lea
8、ds to another view.,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.,9,6,Discrete structure: The electromagnetic field may be thou
9、ght 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,普朗克的关系式将光子的能量E及其频率n 通过下式联系起来,10,6,where
10、 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 th
11、at only the frequency of the radiation is relevant in ejecting electrons.1,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。,11,7,This quantum picture of the electromagnetic field has proved very successful, giving rise to quantum electrodynamics, a quantum field theory describing the i
12、nteraction 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，一种描述电磁辐射与带电物体之间的相互作用的量子场理论。,12,8 Dynamics of the electromagnetic field,In the past, electrically charged objects were thought to produce two types of field associated with their charge property. An electric fie
13、ld 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.,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（
14、和电场）。,13,8,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。,14,9,Once this electromagnetic field has been produced from a given charge distribution, other charged object
15、s 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
16、.3,电磁场中其它带电体就会受到一个力的作用,如果另外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。,15,9,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.,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。,Part II,Microstrip Antenna,17,New Words,18,New Words,19,1,In telecommunication, there are several types of microstrip antennas (also known as printed antenna
