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Author: Lou Frenzel
作者：Lou Frenzel
Radio waves should really be called electromagnetic or EM wavessimply because they consist of a magnetic field and an electricfield. A signal from a transmitter applied to an antenna generatesthe fields. The antenna is the transducer and interface to freespace.
无线电波应该称作电磁波或者简称为EM波，因为无线电波包含电场和磁场。来自发射器、经由天线发出的信号会产生电磁场，天线是信号到自由空间的转换器和接口。
As it turns out, an electromagnetic field’s characteristicschange depending on the distance from the antenna. This varyingfield is typically divided into two segments—the near field and thefar field. A good knowledge of their differences goes a long waytoward understanding radio-wave propagation.
因此，电磁场的特性变化取决于与天线的距离。可变的电磁场经常划分为两部分——近场和远场。要清楚了解二者的区别，就必须了解无线电波的传播。
Electromagnetic Waves
电磁波
Figure 1 shows how a classic half-wave dipole antenna createsthe electric and magnetic fields. The transmitted signal is amodulated sine-wave voltage alternating in polarity, producing anelectric (E) field between the antenna elements that switchespolarity each half cycle. Current in the antenna elements producesa magnetic (H) field that changes orientation each half cycle. Thefields are at right angles to one another.
图1展示了典型的半波偶极子天线是如何产生电场和磁场的。转发后的信号被调制为正弦波，电压呈极性变化，因此在天线的各元件间生成了电场，极性每半个周期变换一次。天线元件的电流产生磁场，方向每半个周期变换一次。电磁场互为直角正交。
1. The electromagnetic field around a half wave dipole consistsof an electric (E) field (a) and a magnetic (H) field (b). Thefields are spherical and cut across one another at rightangles.
1. 围绕着半波偶极子的电磁场包括一个电场(a)和一个磁场(b)。电磁场均为球形且互成直角
The fields around the antenna are spherical or curved,especially near the antenna. As these fields travel out from theantenna, rounding becomes less pronounced, turning more planar incharacter. The receiving antenna usually perceives a planarwave.
天线旁边的磁场呈球形或弧形，特别是距离天线近的磁场。这些电磁场从天线向外发出，越向外越不明显，特性也逐渐趋向平面。接收天线通常接收平面波。
Though the fields exist around the antenna, they propagate awayfrom the antenna perpendicular to the two fields (Fig. 2). At somepoint beyond the antenna, the fields detach themselves into packetsof energy and propagate independently. In fact, they support andregenerate one another along the way. This “independent” wave isthe actual radio wave.
虽然电磁场存在于天线周围，但他们会向外扩张(图2)，超出天线以外后，电磁场就会自动脱离为能量包独立传播出去。实际上电场和磁场互相产生，这样的“独立”波就是无线电波。
2. At a distance from the antenna, the E and H fields areessentially planar and intersect at right angles. Note thedirection of propagation, which is perpendicular to both fields. At(a) the direction of propagation is perpendicular to the fieldlines shown either into or out of the page. In (b) the magneticfield lines are coming out of the page. You can picture them aslines with arrow points shown as dots.
2.距离天线一定范围内，电场和磁场基本为平面并以直角相交。注意传播方向和电磁场均成直角。在(a)图中，传播方向和电磁场线方向成正交，即垂直纸面向内或向外。在(b)图中，磁场线垂直纸面向外，如图中圆圈所示。
The Near Field
近场
There’s seemingly no formal definition for the near field—itdepends on the type of application and the antenna. The most agreedupon definition submits that the near field is less than onewavelength (λ) from the antenna. Wavelength in meters is givenby:
对近场似乎还没有正式的定义——它取决于应用本身和天线。通常，近场是指从天线开始到1个波长(λ)的距离。波长单位为米，公式如下：
λ = 300/fMHz
λ = 300/fMHz
One readily acknowledged distance from the antenna of the nearfield is calculated as:
因此，从天线到近场的距离计算方法如下：
λ/2π = 0.159λ
λ/2π = 0.159λ
Figure 3 shows the radiated sinusoidal wave as well as the nearand far fields. The near field is generally said to be divided intotwo areas, the reactive and the radiative. In the reactive area,the E and H fields are the strongest and can be measuredseparately. One field or the other will likely dominate, dependingon antenna type. A loop antenna, for example, is dominated by themagnetic (H) field. The loop antenna appears to be the primary as atransformer because of the large magnetic field it generates.
图3标出了辐射出的正弦波和近场、远场。近场通常分为两个区域，反应区和辐射区。在反应区里，电场和磁场是最强的，并且可以单独测量。根据天线的种类，某一种场会成为主导。例如环形天线主要是磁场，环形天线就如同变压器的初级，因为它产生的磁场很大。
3. Boundaries of the near and far fields are shown with respectto wavelengths at the operating frequency. The antenna is assumedto be at the left and beginning of the wave.
3.近场和远场的边界、运行频段的波长如图所示。天线应位于正弦波左侧起始的位置。
In the radiative area, the fields begin to radiate. Itrepresents the beginning of the far field. In the near field, thestrength of the fields varies inversely with the cube of thedistance from the antenna (1/r3).
辐射区内，电磁场开始辐射，标志着远场的开始。场的强度和天线的距离成反比(1/ r3)。
The transition zone in Figure 3 refers to the somewhat undefinedarea between the near and far fields. (Some models don’t define atransition zone.) In this figure, the far field begins at adistance of 2λ and beyond.
图3所示的过渡区是指近场和远场之间的部分(有些模型没有定义过渡区)。图中，远场开始于距离为2λ的地方。
The Far Field
远场
Much like the near field, definitions vary on the beginning ofthe far field. Some say 2λ, while others insist that it is 3λ or10λ from the antenna. Another definition indicates that it startsat 5λ/2π, while still another says that it depends on the largestdimension of the antenna D or 50D2/λ.
和近场类似，远场的起始也没有统一的定义。有认为是2 λ，有坚持说是距离天线3 λ或10λ以外。还有一种说法是5λ/2π，另有人认为应该根据天线的最大尺寸D，距离为 50D2/λ。
Then there are those who claim that this fuzzy boundary betweennear and far fields begins at 2D2/λ. Others will say that the farfield begins where the near field leaves off, or as indicatedearlier, λ/2π.
还有人认为近场远场的交界始于2D2/λ。也有人说远场起始于近场消失的地方，就是前文提到的λ/2π。
The far field is the real radio wave. It propagates throughspace at a speed of just about 300 million meters per second, whichis the speed of light or nearly 186,400 miles per second. The E andH fields support and regenerate one another as their strengthdecreases inversely as the square of the distance (1/r2). Maxwelldescribed this phenomenon in his infamous equations.
远场是真正的无线电波。它在大气中以3亿米/秒的速度，即接近18.64万英里/秒的速度传播，相当于光速。电场和磁场互相支持并互相产生，信号强度和距离平方成反比(1/r2)。麦克斯韦在其著名的公式中描述了这一现象。
Maxwell’s Equations
麦克斯韦方程组
In the late 1870s, before the invention of radio, Scottishphysicist James Clerk Maxwell predicted the arrival ofelectromagnetic waves. Using the laws known at the time fromAmpere, Faraday, Ohm, and others, he came up with a set ofequations that illustrates how one type of field generates theother, and as they propagate, the two coexist in a supportingrelationship. In the late 1880s, German physicist Heinrich Hertzproved Maxwell’s theory.
19世纪70年代末，在无线电波发明之前，苏格兰物理学家詹姆斯?克拉克?麦克斯韦预测出了电磁波的存在。他综合了安培、法拉第和欧姆等人的定律，制定了一套方程表达电磁场是如何相互产生和传播的，并断定电场和磁场互相依存、互相支持。19世纪80年代末，德国物理学家海因里希?赫兹证明了麦克斯韦的电磁场理论。
Maxwell developed four basic equations that show therelationship of the electric and magnetic fields as they vary withtime. Basically, the electric field changing over time appears toproduce charges in motion or current flow that sets up a magneticfield. Other equations say that as the magnetic field varies it canset up an electric field. The waves propagate through space bythemselves after leaving the antenna. Those equations aren’t shownhere, but you may recall that they involve partial differentialequations.
麦克斯韦创造了四个基本方程，表达电场、磁场和时间之间的关系。电场随时间推移产生移动电荷，也就是电流，从而产生磁场。另一组方式是说，变化的磁场可以产生电场。天线发出的电磁波在空间中自行传播。本文没有列出这些方程组，但你应该记得包含一些不同的方程。
Applications
应用
The far field propagates through space, and its strength isdefined by the Friis formula:
远场在空间中传播的强度变化由Friis公式决定：
Pr = PtGrGtλ2/16π2r2
Pr = PtGrGtλ2/16π2r2
where Pr = power received; Pt = power transmitted; Gr = receiveantenna gain (power ratio); Gt = transmit antenna gain (powerratio); and r = range or distance from antenna. The formula isvalid for free space, line of sight, with no obstructions.
公式中，Pr =接收功率；Pt =发射功率；Gr = 接收天线增益(功率比)；Gt=发射天线增益(功率比)；r=到天线的距离。公式在视线所及的无障碍开阔空间中适用。
Two important facts arise in this discussion. The received powervaries inversely with the square of the range r. It also varieswith the square of the wavelength, meaning that longer waves atlower frequencies travel farther. For example, a 900-MHz signalwill travel farther than a 2.4-GHz signal for similar power andantenna gains. This expression can be used to analyze all modernwireless applications in terms of approximating signalstrength.
这里有两个问题需要讨论。接收功率和距离r的平方成反比，和波长的平方成正比，也就是说，波长较长、频率较低的电磁波传的更远。例如，同等的功率和天线增益下，900MHz的信号会比2.4GHz的信号传播得更远。这一公式也常常用它来分析现代无线应用的信号强度。
To accurately observe signal propagation, one must plot theantenna’s radiation pattern in the far field. In the reactive zoneof the near field, the receiving antenna may interact with thetransmitting antenna via capacitive or inductive coupling and thusgive false results. On the other hand, it’s been shown that aradiation pattern in the near field can be accurately plotted ifspecial measurement equipment is available.
为了准确测量信号的传播，还必须了解天线在远场的辐射模式。在近场的反应区里，接收天线可能会和发射天线会由于电容和电感的耦合作用互相干扰，造成错误的结果。另一方面，如果有特定的测量仪器，近场的辐射模式就可以准确测量。
The near field has also proved useful in communications. Thismode is used for applications such as radio-frequencyidentification (RFID) and near-field communications (NFC).
近场在通信领域也很有用。近场模式可以用于射频识别(RFID)和近场通信(NFC)。
RFID is the electronic equivalent of bar coding. A thin tagcontaining a chip that integrates memory and specific electroniccode is attached to the item to be identified, tracked, orotherwise processed. The tag, which also includes a passivetransceiver, is passed near a “reader” transceiver that emits astrong RF signal picked up by the tag. Both reader and tag antennasare usually loops serving as the primary and secondary of atransformer.
RFID是条形码的电子版，它是一个内部有芯片的很薄的标签，其中芯片集成了存储和特定的电子代码，可以用作识别、最总或其他用途。标签还包含一个被动收发器，在接近“阅读器”的时候，由阅读器发出的很强的RF信号就会被标签识别。阅读器和标签的天线都是环形天线，相当于变压器的初级和次级。
The signal picked up by the tag is rectified and filtered intodc, which provides power to the tag memory and transmitter. Thetransmitter then sends the code to the reader for identificationand further processing. Active tags using a battery sometimesextend the read range beyond the near field. RFID tags come indifferent frequency ranges, such as 125 kHz, 13.56 MHz, and 900MHz.
由标签识别的信号经过整流滤波转换成直流，为标签存储和转发供能。发射器将代码发送到阅读器上，用于识别和处理。主动标签有时会用到电池，将感应距离延长到近场以外的地方。RIFD标签的频率范围各不相同，有125kHz、13.56MHz和900MHz。
At 900 MHz, the wavelength is:
在900MHz，波长为：
λ = 300/fMHz
λ = 300/fMHz
λ = 300/900 = 0.333 meter or 33.33 cm
λ = 300/900 = 0.333 米或 33.33 cm
Subsequently, the near field is calculated as:
因此根据近场距离计算公式：
λ/2π = 0.159λ = 0.159(0.333) = 0.053 meter (about 2 inches)
λ/2π = 0.159λ = 0.159(0.333) = 0.053 米 (约2英寸)
Read ranges usually extend somewhat beyond this point.Therefore, it may actually spill into the far field at thisfrequency.
感应距离通常超过这一数字，所以这一频率下距离实际上也延伸到了远场。
NFC also employs a memory and special coding similar to that ofa credit card. An internal transceiver, usually battery powered,can transmit the code to a reader. It also uses the near field asthe read range, and it’s typically only inches. The NFC frequencyis 13.56 MHz, representing a wavelength of:
NFC也采用了存储和类似于信用卡的特定代码。电池驱动的内部转发器可以把代码发射到阅读器上。NFC也使用近场，范围一般为几英寸。NFC的频率为13.56MHz，因此波长为：
λ = 300/fMHz
λ = 300/fMHz
300/13.56 = 22.1 meters or 72.6 feet
300/13.56 = 22.1 米或 72.6 英尺
The near field is within:
近场距离为不超过：
λ/2π = 0.159λ = 0.148(72.6) = 11.5 feet
λ/2π = 0.159λ = 0.148(72.6) = 11.5 英尺
Because less power is used, the actual read range is rarelygreater than a foot.
因为电量消耗低，实际的感应距离很少超过1英尺。
NFC is expected to be the technology to implement the “digitalwallet.” With this application, consumers make payments usingNFC-enabled smart phones rather than a credit card.
NFC是部署“电子钱包”所使用的技术。通过电子钱包，消费者可以无需信用卡，而用支持NFC的智能手机进行付款。
References
参考资料
1. Cheung, W. S. and Levien, F. H., Microwaves Made Simple,Principles and Applications, Artech House Inc., 1985.
Cheung, W. S. 和Levien, F. H.，微波测量原理和应用，Artech出版社，1985
2. Occupational Safety & Health Administration,Electromagnetic Radiation: Field Memo, 1990.
职业安全与健康管理，电磁辐射：现场笔记，1990
3. Straw, R. D. (Editor), The ARRL Antenna Book, American RadioRelay League, 1997-8.
Straw R. D. (编辑)，天线手册，美国无线电传播联盟，1997-8
4. Volakis, J.L., Antenna Engineering Handbook, 4th edition,McGraw-Hill, 2007.
Vokakis,J.L.,天线工程手册，第四版，McGraw-Hill出版社，2007
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