电磁传播中英文.doc

文摘
This paper attempts to provide some insight into the nature of radio propagation in that part of the spectrum (upper VHF to microwave) used by experimenters for high-speed digital transmission. 本文试图提供一些深入的无线电传播的本质中这一部分的频谱(上部甚高频微波)使用高速数字传输实验人员。It begins with the basics of free space path loss calculations, and then considers the effects of refraction, diffraction and reflections on the path loss of Line of Sight (LOS) links. 它开始于基本的自由空间路径损耗的计算,然后考虑折射的影响、衍射、反思的路径损耗的视线(LOS)链接。The nature of non-LOS radio links is then examined, and propagation effects other than path loss which are important in digital transmission are also described.non-LOS无线链路的本质是然后检查,其他的作用和传播路径损耗这很重要,数字传输也进行了描述。
Introduction介绍
The nature of packet radio is changing. 小包收音机的性质正在发生变化。As access to the Internet becomes cheaper and faster, and the applications offered on the "net" more and more enticing, interest in the amateur packet radio network which grew up in the 1980s steadily wanes. 访问互联网变得更快、更便宜,而应用程序提供的“净”更多,更吸引人,兴趣业余无线电网络数据包成长于1980年代逐步减弱。To be sure, there are still pockets of interest in some places, particularly where some infrastructure to support speeds of 9600 bps or more has been built up, but this has not reversed the trend of declining interest and participation. 可以肯定的是,还有人的钱包在一些地方,尤其是在一些基础设施,以支持时速为9600个基点以上建立起来了,但是这并没有扭转趋势下降的兴趣和参与。Nevertheless, there is still lots of interest in packet radio out there - it is simply becoming re-focused in different areas. 然而,仍然有很多兴趣分组无线——它仅仅是在不同的地区变得严格。Some applications which do not require high speed, and can take advantage of the mobility that packet radio can provide, have found a secure niche - APRS is a good example. 有些应用程序不需要高速度,并可以利用机动性,小包收音机可以提供,找到了一个安全的位置——APRS就是一个很好的例子。Interest is also high in high-speed wireless transmission which can match, or preferably exceed, landline modem rates. 利息也有很高的高速无线传输可以匹配,或者最好超过、有线调制解调器率。With a wireless link, you can have a 24-hour network connection without the need for a dedicated line, and you may also have the possibility of portable or mobile operation. 与一个无线连接,你可以有一个24小时的网络连接,而不需要使用专用线路,你或许也有可能性的便携式或移动操作。Until recently, most people have considered it to be just too difficult to do high-speed digital. 直到最近,大多数人都认为它非常难做高速数字。For example, the WA4DSY 56 Kbps RF modem has been available for ten years now, and yet only a few hundred people at most have put one on the air. 例如,WA4DSY 56 Kbps射频现代已有十年,然而现在只有几百人在大多数都把一个放在空气。With the new version of the modem introduced last year, 56 Kbps packet radio has edged closer to plug 'n play, but in the meantime, landline modem data rates have moved into the same territory. 与新版本的调制解调器在去年推出的56 Kbps包广播已经更加接近堵塞的氮发挥,但与此同时,许多有线调制解调器的数据速率已经进入相同的境地。What has really sparked interest in high-speed packet radio lately is not the amateur packet equipment, but the boom in spread spectrum (SS) wireless LAN (WLAN) hardware which can be used without a licence in some of the ISM bands. 真正的兴趣在高速分组无线最近不是业余包设备,但繁荣的扩频(SS)无线局域网(WLAN)硬件可以使用无证在一些ISM的乐队。The new WLAN units are typically integrated radio/modem/computer interfaces which mimic either ethernet interfaces or landline modems, and are just as easy to set up. 新的WLAN单位通常集成无线/现代/计算机接口模拟要么以太网接口或有线调制解调器,也同样容易设置。Many of them offer speeds which landline modem users can only dream of. 很多提供有线调制解调器速度,用户只能梦想。TAPR and others are working on bringing this type of SS technology into the amateur service, where it can be used on different bands, and without the Effective Radiated Power (ERP) restrictions which exist for the unlicenced service. TAPR和其他公司正致力于将这种类型的SS技术到业余服务,在那里可以用于在不同的乐队,又没有有效辐射功率(ERP)的限制而存在unlicenced服务。This technology will be the ticket to developing high-speed wireless LANs and MANs which, using the Internet as a backbone, could finally realize the dream of a high-performance wide-area AMPRnet which can support the applications (WWW, audio and video conferencing, etc.) that get people excited about computer networking these days.这项技术将会成为机票发展高速无线局域网和芒,使用互联网作为骨干,终于可以实现梦想的高性能广域AMPRnet可以支持应用程序(WWW、音频和视频会议等),让人们兴奋这些天电脑网络。

Although the dream as stated above is somewhat controversial, the author believes it represents the best hope of attracting new people to the hobby, providing a basis for experimentation and training in state-of-the-art wireless techniques and networking, and, ultimately, retaining spectrum for the amateur radio service. 虽然梦想是像上面说的是有点争议的,作者认为它代表了最佳希望吸引新人们的喜好提供依据实验和培训在最先进的无线技术和网络,最终保留业馀无线电频谱服务。One problem is that most of the people attracted to using digital wireless techniques have little or no background in RF. 一个问题是,大部分的人吸引到使用数字无线技术有很少或没有背景在射频(RF)。When it comes to setting up wireless links which will work over some distance, they tend to lack the necessary knowledge about antennas, transmission lines and, especially, the subtleties of radio propagation. 当涉及到建立无线连接将工作有一段距离,他们往往缺乏必要的知识对天线、输电线路,尤其是无线电传播的微妙之处。This paper deals with the latter area, in the hopes of providing this new crop of digital experimenters with some tools to help them build wireless links which work.本文论述了后者区域,希望能提供这一批新的数字实验者带有一些工具来帮助他们建立无线连接而工作。
The main emphasis of this paper is on predicting the path loss of a link, so that one can approach the installation of the antennas and other RF equipment with some degree of confidence that the link will work. 本文的重点是在预测路径损耗的链接,所以可以方法安装天线和其他射频设备提供了一定程度的信心,链接将工作。The focus is on acquiring a feel for radio propagation, and pointing the way towards recognizing the alternatives that may exist and the instances in which experimentation may be fruitful. 重点是获得一个感觉无线电传播,并指明了方向接近承认可能存在的可选方案,实验的实例可能是卓有成效的。We'll also look at some propagation aspects which are of particular relevance to digital signaling.我们也会看下传播方面密切相关的数字信号。
Estimating Path Loss估计路径损耗
The fundamental aim of a radio link is to deliver sufficient signal power to the receiver at the far end of the link to achieve some performance objective. 的根本目的就是提供无线电连接到接收方充分信号功率尽头的链接来实现一些性能目标。For a data transmission system, this objective is usually specified as a minimum bit error rate (BER). 对于一个数据传输系统,这个目标通常是指定为一个最低误码率(BER)。In the receiver demodulator, the BER is a function of the signal to noise ratio (SNR). 在接收端解调器、误码是一个函数的信噪比(SNR)。At the frequencies under consideration here, the noise power is often dominated by the internal receiver noise; however, this is not always the case, especially at the lower (VHF) end of the range. 频率在考虑在这里,噪音的权利常常由内部接收机噪声;然而,这并非总是如此,特别是在低(高频)的范畴。In addition, the "noise" may also include significant power from interfering signals, necessitating the delivery of higher signal power to the receiver than would be the case under more ideal circumstances (i.e., back-to-back through an attenuator). 此外,“噪音”还可能包括重要的权力的干扰信号,需要交付更高的信号接收能力会比更理想的环境下的情况(例如,通过一个背靠背衰减器)。If the channel contains multipath, this may also have a major impact on the BER. 如果通道包含路径”,这可能也具有重要影响的误码率。We will consider multipath in more detail later - for now, we will focus on predicting the signal power which will be available to the receiver.我们将在后面详细考虑多路径——现在,我们将专注于预测信号功率可提供给接收者。
Free Space Propagation自由空间传播
The benchmark by which we measure the loss in a transmission link is the loss that would be expected in free space - in other words, the loss that would occur in a region which is free of all objects that might absorb or reflect radio energy. 我们衡量的基准损失在传输环节是损失,预期在自由的空间——换句话说,损失,可能发生在一个地区免费提供的所有对象,可以吸收或反射的无线电能量。This represents the ideal case which we hope to approach in our real world radio link (in fact, it is possible to have path loss which is less than the "free space" case, as we shall see later, but it is far more common to fall short of this goal).这代表了理想的情况下,我们希望在我们的现实世界的方法无线电连接(事实上,它可能有路径损耗低于“自由空间”情况下,如同我们将看到后来,但更为常见,低于这个目标)。
Calculating free space transmission loss is quite simple. 计算空闲空间传输损失是相当简单的。Consider a transmitter with power Pt coupled to an antenna which radiates equally in all directions (everyone's favorite mythical antenna, the isotropic antenna). 考虑一个发射器与权力Pt耦合天线辐射同样在各个方向(人人都喜欢的神秘的天线,各向同性天线)。At a distance d from the transmitter, the radiated power is distributed uniformly over an area of 4在距离从发射机,辐射功率分布均匀占地4d2 (i.e. the surface area of a sphere of radius d), so that the power flux density is:d2(即的面积为球心的球面d),使电力通量密度是:
(1)(1)
The transmission loss then depends on how much of this power is captured by the receiving antenna. 然后主要就取决于传输损失多少权力是所捕获的接收天线。If the capture area, or effective aperture of this antenna is Ar, then the power which can be delivered to the receiver (assuming no mismatch or feedline losses) is simply如果捕获区域,或有效孔径的这种天线Ar,那么电力的可交付给接收机(假设没有不匹配或馈线损失)是简单的
(2)(2)
For the hypothetical isotropic receiving antenna, we have对于各向同性假设的接收天线,我们有
(3)(3)
Combining equations (1) and (3) into (2), we have结合方程(1)和(3)(2),我们有
(4)(4)
The free space path loss between isotropic antennas is Pt /自由空间各向同性之间的路径损耗天线Pt /Pr. Since we usually are dealing with frequency rather than wavelength, we can make the substitution = c/f (where c, of course, is the speed of light) to get因为我们通常是公关。处理频率而不是波长,我们可以进行替换=写成(c,当然,是光的速度)来获得
(5)(5)

This shows the classic square-law dependence of signal level versus distance. 这显示了典型的square-law信号电平的依赖比距离。What troubles some people when they see this equation is that the path loss also increases as the square of the frequency. 什么令一些人当他们看到这个方程式的路径损耗也增加的平方的频率。Does this mean that the transmission medium is inherently more lossy at higher frequencies? 这是否意味着传输介质本质上是更有损在更高的频率?While it is true that absorption of RF by various materials (buildings, trees, water vapor, etc.) tends to increase with frequency, remember we are talking about "free space" here. 虽然这是事实,通过��种材料的吸收射频(建筑、树木、水蒸气等)往往会增加与频率,记得我们在谈论这里的“自由空间”。The frequency dependence in this case is solely due to the decreasing effective aperture of the receiving antenna as the frequency increases. 在这种情况下,频率依赖性单纯是由于减少有效孔径的接收天线频率增加。This is intuitively reasonable, since the physical size of a given antenna type is inversely proportional to frequency. 这在直觉上是合理的,因为一个给定的物理尺寸天线类型是成反比的频率。If we double the frequency, the linear dimensions of the antenna decrease by a factor of one-half, and the capture area by a factor of one-quarter. 如果我们两倍的频率,线性尺度天线降低一半的因素,以及捕获一个因子四分之一的地区。The antenna therefore captures only one-quarter of the power flux density at the higher frequency versus the lower one, and delivers 6 dB less signal to the receiver. 天线因此捕捉仅有四分之一的电力通量密度,在更高的频率与较低的人,并提供6分贝较少的信号到接收方。However, in most cases we can easily get this 6 dB back by increasing the effective aperture, and hence the gain, of the receiving antenna. 然而,在大多数情况下,我们可以很容易地得到这6分贝回来通过增加有效孔径,并因此获得的接收天线。For example, suppose we are using a parabolic dish antenna at the lower frequency. 例如,假设我们使用抛物面碟形天线在更低的频率。When we double the frequency, instead of allowing the dish to be scaled down in size so as to produce the same gain as before, we can maintain the same reflector size. 当我们两倍的频率,而不允许在菜被缩减规模,从而产生相同的获得像之前一样,我们可以保持相同的反射器的大小。This gives us the same effective aperture as before (assuming that the feed is properly redesigned for the new frequency, etc.), and 6 dB more gain (remembering that the gain is with respect to an isotropic or dipole reference antenna at the same frequency). 这给了我们相同的有效孔径之前(假设该提要是正确地重新设计以新频率,等等),以及6分贝更多获得(记住,收获的是有关一个各向同性、偶极子天线引用相同的频率)。Thus the free space path loss is now the same at both frequencies; moreover, if we maintained the same physical aperture at both ends of the link, we would actually have 6 dB less path loss at the higher frequency. 因此,自由空间都是相同的路径损耗在两个频率;此外,如果我们保持相同的物理孔径的两端的链接,我们将真正有6分贝路径损耗少在更高的频率。You can picture this in terms of being able to focus the energy more tightly at the frequency with the shorter wavelength. 你可以想象这方面能够将能量��紧密的频率与较短的波长。It has the added benefit of providing greater discrimination against multipath - more about this later.它有更多的好处,提供更大的歧视——会在本文稍后的路径。
The free space path loss equation is more usefully expressed logarithmically:自由空间路径损耗方程是更有用的表达了对数:
(f in MHz, d in km) (6a)(f在兆赫,d公里)(6)

or或
(f in MHz, d in miles) (6b)(f在兆赫,d英里)(6 b)

This shows more clearly the relationship between path loss and distance: path loss increases by 20 dB/decade or 6 dB/octave, so each time you double the distance, you lose another 6 dB of signal under free space conditions.这显示了更清晰的路径损耗之间的关系和距离:路径损失增加了20分贝/十年或6分贝/倍频程,所以你每次两倍的距离,你失去了另一个6分贝的信号在自由空间条件。
Of course, in looking at a real system, we must consider the actual antenna gains and cable losses in calculating the signal power Pr which is available at the receiver input:当然,在看一个真正的系统,我们必须考虑天线收益和电缆损失计算信号功率的公关是可用的接收器输入:
(7)
where在
Pt = transmitter power output (dBm or dBW, same units as Pr)Pt =发射机功率输出(dBm或大功率,同样的单位公关)
Lp = free space path loss between isotropic antennas (dB)Lp =自由空间各向同性之间的路径损耗天线(dB)
Gt = transmit antenna gain (dBi)Gt =传输天线增益(dBi)
Gr = receive antenna gain (dBi)Gr =接收天线增益(dBi)
Lt = transmission line loss between transmitter and transmit antenna (dB)Lt =输电线路损失发射器和天线之间传输(dB)
Lr = transmission line loss between receive antenna and receiver input (dB)Lr =输电线路损失接收天线和接收方之间的输入(dB)
A table of transmission line losses for various bands and popular cable types can be found in the Appendix.一个表传输线损失为各种乐队和受欢迎的电视类型中可以找到的附录。
Example 1.示例1。Suppose you have a pair of 915 MHz WaveLAN cards, and want to use them on a 10 km link on which you believe free space path loss conditions will apply. 假设你有一双915 MHz WaveLAN卡,并想使用它们在一个10公里链接在你相信自由空间路径损耗状况将应用。The transmitter power is 0.25 W, or +24 dBm. 发射机功率为0.25 W或+ 24 dBm。You also have a pair of yagi antennas with 10 dBi gain, and at each end of the link, you need about 50 ft (15 m) of transmission line to the antenna. 你也有一双八天线进行10 dBi增益、两端的链接,你需要大约50英尺(15米)的输电线路到天线。Let's say you're using LMR-400 coaxial cable, which will give you about 2 dB loss at 915 MHz for each run. 让我们假设你使用LMR - 400同轴电缆,这将会给你约2 dB损失在915 MHz的每次运行。Finally, the path loss from equation (6a) is calculated, and this gives 111.6 dB, which we'll round off to 112 dB. 最后,路径损耗从方程(6)计算,这让111.6分贝,这我们会圆滑到112分贝。The expected signal power at the receiver is then, from (7):预期的信号功率在接收方是之后,从(7):

According to the WaveLAN specifications, the receivers require -78 dBm signal level in order to deliver a low bit error rate (BER). 根据WaveLAN规格,接收器需要-78 dBm信号电平,以交付一个较低的误码率(BER)。So, we should be in good shape, as we have 6 dB of margin over the minimum requirement. 所以,我们应该不错,因为我们有6分贝的在领先的最低要求。However, this will only be true if the path really is equivalent to the free space case, and this is a big if! 然而,这只会是真的如果路径真的相当于自由空间的情况下,这是一个很大的如果!We'll look at means of predicting whether the free space assumption holds in the next section.我们会看看意味着空闲空间的预测是否假设认为在接下来的部分中。
Path Loss on Line of Sight Links路径损耗在视线范围内的链接
The term Line of Sight (LOS) as applied to radio links has a pretty obvious meaning: the antennas at the ends of the link can "see" each other, at least in a radio sense. 术语的视线(LOS)是应用于无线电连接有一个非常明显的意义:天线末端的链接可以“看见”彼此,至少在一个电台意义。In many cases, radio LOS equates to optical LOS: you're at the location of the antenna at one end of the link, and with the unaided eye or binoculars, you can see the