【正文】 ve an enormously useful and popular subject. Maxwell’s equations laid the foundation and laws of the science of electromagic, of which the field of RF and microwave is a small subset. Due to the exact and allenpassing nature of these laws in predicting electromagic phenomena, along with the great body of analytical and experimental investigations performed since then, we can consider the field of RF and microwave engineering a “mature discipline” at this time. Applications of Maxwell’s Equations As indicated earlier in Chapter 2, Fundamental Concepts in Electrical and Electronics Engineering, standard circuit theory can neither be use at RF nor particularly at microwave frequencies. This is because the dimensions of the device or ponents are parable to the wavelength, which means that phase of an electrical signal (., a current or voltage) changes significantly over the physical length of the device or ponent. Thus use of Maxwell’s equations at these higher frequencies bees imperative. In contrast, the signal wavelengths at lower frequencies are so much larger than the device or ponent dimensions, that these are negligible variation in phase across the dimensions of the circuit. Thus Maxwell’s equations simplify into basic circuit theory, as covered in Chapter 3, Mathematical Foundation for Understanding Circuits. At the other extreme of the frequency range lies the optical field, where the wavelength is much smaller than the device or circuit dimensions. In this case, Maxwell’s equations simplify into a subject monly referred to as geometrical optics, which treats light as a ray traveling on a straight line. These optical techniques may be applied successfully to the analysis of very high microwave frequencies (., high millimeter wave range),where they are referred to as “quasioptical.” Of course, it should be noted that further application of Maxwell’s equation leads to an advanced field of optics called “physical apices or Fourier optics,” which treats light as a wave and explains such phenomena as diffraction and interference, where geometrical optics fails pletely. The important conclusion to be drawn from this discussion is that Maxwell’s equations present a unified theory of analysis for any system at any frequency, provided we use appropriate simplifications when the wavelengths involved are much larger, parable to, or much smaller than the circuit dimensions. Properties of RF and Microwaves An important property of signals at RF, and particularly at higher microwave frequencies, is their great capacity to carry information. This is due to the large bandwidths available at these high frequencies. For example, a 10 percent bandwidth at 60MHz carrier signal is 6MHz, which is approximately one TV channel of information。他還以純數(shù)學(xué)觀點(diǎn)從理論上預(yù)言存在電磁波傳播現(xiàn)象，以及光也是電磁波的一種形式，這在當(dāng)時(shí)是 全新的概念。 1897 年，瑞利從數(shù)學(xué)上證明電磁波也可以在圓形和方形的波導(dǎo)中傳播，無論是橫波顛簸還是橫磁波，都存在無窮多種可能的模式，每一種都有自己特定的截止頻率。由于麥克斯韋定律可能準(zhǔn)確預(yù)測(cè)各種電磁現(xiàn)象，加之后續(xù)進(jìn)行的大量的理論分析和實(shí)驗(yàn)研究工作，可以說射頻 /微波工程是一門成熟的學(xué)科。 以上光學(xué)方法可成功運(yùn)用于高頻微波分析，這時(shí)稱其為“準(zhǔn)光學(xué)”。 雷達(dá)是很重要的軍民兩用設(shè)備，它主要依靠雷達(dá)截面，即目標(biāo)的有效反射面積來發(fā)現(xiàn)目標(biāo)，微波段是使用雷達(dá)的理想頻段。 但是應(yīng)用射頻 /微波的多數(shù)系統(tǒng)都是利大于弊，這也促使工程師愿意選擇高頻設(shè)計(jì)。 雷達(dá) 雷達(dá)應(yīng)用包括防空、飛機(jī)船舶導(dǎo)航、智能武器系統(tǒng)、治安警防、天氣預(yù)報(bào)、避障和成像等。