大孔径阵列可以提供了特殊的屏蔽的挑战
罗恩·布鲁尔,Laird技术,特拉华州水口,PA
可以使用屏蔽系统或印刷电路板一级保护敏感设备或电路内部和外部射频来源。大型系统可以放置在屏蔽室,甚至保护建筑。然而,无论外壳的大小,以同样的方式工作,遭受同样的问题。短暂,盾牌工作因为辐射场和屏蔽阻抗结果的差异在一些射频能量目标圈地的反射(R)的表面,在一些能量被吸收(A),因为它穿过屏蔽材料。剩余的能源出口围栏的另一边。入射电场强度的比例进入场强的盾牌退出另一方面是屏蔽效能。在更高的频率,一个封闭的屏蔽效能是由吸收:(dB) = Ka t√(µσF) (dB) =吸收损失,Ka = 3.338密耳(t)或131.4毫米(t), t =厚度(千或毫米),µ=相对渗透率、相对电导率σ=,F =在兆赫频率。吸收具有重要意义和继续增加频率。事实上,衰减变得如此之大,高频屏蔽通常并不是由屏蔽材料。例如,一个小型电子围栏由铝(µ= 1,σ= 0.64)会有近似的厚度40毫升。 At a frequency of 100 MHz, the absorption loss is 26.7 dB per mil of thickness, and the enclosure attenuation would be 1068 dB. At 1000 MHz, its absorption loss would be 3378 dB! Of course, we aren’t able to measure attenuation that large. Measurements are limited to about 140 dB. Nevertheless, shielding affords suppression that does not attach to the circuit, that is capable of providing attenuation levels exceeding 3000 dB, and that provides attenuation levels that increase with frequency. Seemingly, shielding could be the almost-perfect suppression component. Unfortunately, the circuits requiring shielding are not so perfect! As circuits generate heat because of their inefficiencies, the temperature internal to the shield rises; and the resultant heat must be removed. There are three ways of eliminating this heat: thermal radiation, conduction, and convection. The most effective is convection, which can be either natural or forced air using internal exhaust fans. The mechanical, fan-based approach to heat exhaustion is highly effective and thus allows greater circuit density, but it requires holes in the shield. Further, to provide the necessary airflow, one hole is generally not enough. An array of holes (apertures) is needed. These large aperture arrays degrade the shielding and present special design difficulties because of mechanical variations. The bigger the aperture, the poorer the shielding; and in many cases, in spite of the material being used for the enclosure, shielding effectiveness (SE) does not exceed 30 to 40 dB! A simple expression for the worst-case attenuation of a single hole or slot aperture (that is not penetrated by a conductor) in a thin conducting sheet is: S(dB) = k log (λ/2 L) where S(dB) = slot attenuation k is determined by the shape of the hole and ranges between 20 and 40,1、2、3波长λ=事件,和L =槽长度(<λ/ 2)。截止频率以上的洞(L =λ/ 2)时,屏蔽效能是假定为零(0)。尽管这种假设简化了方程,它是不准确的,因为实际行为的槽孔径相当于一个互补偶极子。注意,偶极子的电磁场模式和槽是相同的,但是他们的电场和磁场颠倒了。上述方程可以解决槽的长度(L)来确定光圈大小,结果在一个给定的衰减。不过,总体而言,光阑应该小于λ/ 50和不应大于λ/ 20。在100 MHz实现可接受的衰减值,例如,孔径应小于10毫米。一般来说,接缝往往是最大的,因此,“最坏的光阑。“很明显,由于接缝不能任何大于外壳本身,圈地越小,越好。此外,较小的附件有更少的渗透;反过来,这些缝隙小。 Still, there is a problem with this simplified approach because holes of equal area can have significantly different shapes. A slot is a good example. Slots have long opening lengths combined with small opening widths. Obviously, a narrow-width slot must be quite long to equal the area of a square. Since the cutoff frequency for RF energy polarized with the slot length is determined only by the length and not by the opening width, the degradation in shielding is proportional to the combined length of the slots and not their area. For this polarization, the width of the slot principally determines the slot bandwidth (Figures 1 and 2). Considerable improvement in aperture shielding effectiveness can be obtained (as λ/2 approaches the value L) by increasing the thickness of the material so that it is equal to, or greater than L—i.e., (t ≥ L). At this point, the opening begins to act as a waveguide being operated below its cutoff frequency. Although it will depend on the propagation mode, for frequencies that are less than F /3, waveguide attenuation is independent of frequency and is given thusly A(dB) ≅ 30 t/LwhereA(dB) = attenuation,t = thickness, andL = length or diameter of theopening. This equation is for one waveguide, and the constant, 30, is an approximation. The actual value is determined by hole shape, and ranges from about 28 to 32. For a t/L ratio of 4, typical of high performance honeycomb shields, the calculated attenuation of one cell is 120 dB. In the frequency range between F /3 and F, the attenuation decreases, reaching 0 dB at F. Even so, the waveguide has significantly greater attenuation in this frequency range than the same size hole in a thin sheet would exhibit. Although there may be considerable variation in the attenuation resulting from manufacturing differences, this characteristic is fundamental to the superiority of honeycomb, which is constructed to create small waveguides assembled in parallel. The shielding relationship (attenuation in dB/ in. for a single aperture) between frequency, hole size, and thickness is shown in Figure 3. As more apertures are added, the enclosure attenuation degrades. A simplified way of calculating the effects of multiple holes is to reduce the attenuation estimate by 20 log n, where n is the number of holes. Two holes would be -6 dB; ten holes would be -20 dB; and so on. Simplistically, it would seem that a large aperture array with many holes would have no attenuation at all. Fortunately, in the near field, the number of holes that can contribute to the reduction in shielding is limited. This limiting effect is determined by the separation between the holes. As the spacing between the holes increases, coherent recombination of the RF energy from the widely spaced holes becomes impossible; and consequently, the holes act independently. Thus the maximum number of holes that determine the attenuation of the array can be approximated by those contained in a circular area (A = πr2)的半径λ/ 2。令人沮丧的是,即便如此,这个近似失败。注意反直觉的语句,如果开口的大小是由更小的,这样更多的包围面积(A),屏蔽将退化。这个结论完全不是那么回事。Bereuter和张这个谬论的综合分析应用于蜂窝屏蔽。4低频应用程序只需要适度的屏蔽,成本最低的方法是使用多孔材料或编织或针织筛查。然而,对于那些关键的应用,要求
荷兰国际集团(ing)高效屏蔽通风板和/或杰出的气流特性在禁区,蜂窝材料结合最好的气流与最好的衰减。蜂窝构造小相邻电气性连接管(小波导)由薄金属箔条并行装配。这能产生一种物质,是大约97%开放区域。蜂窝有更大的方向性特征与洞薄材料相比,但气流通过蜂窝不是动荡。同时,定向特性允许建造蜂巢通风板防滴漏或视觉安全配置。尽管一些屏蔽材料提供的令人印象深刻的人物。(高性能蜂窝可以提供120分贝衰减在18个GHz, 40密耳厚和固体铝访问面板提供了一个吸收亏损3500分贝),安装仍是真正有效屏蔽的关键。任何面板不能充分结合到圈地行为有损天线结构坐在一个洞。接地的面板一度将减少天线效率,甚至可能解决在低频率辐射问题,但不会消除泄漏其他seam。例如,如果120 - db蜂窝不当安装在一个小6。 muffin fan, the perimeter leakage could limit the enclosure attenuation at 100 MHz to about 20 dB! The best installation methods are welding, brazing, soldering or riveting—in that order. But these methods preclude easy access for maintenance and repair. Installing closely spaced threaded fasteners or clamps permits, but does not facilitate, field removal. RF gaskets also play a vital role in shielding effectiveness. With the exception of enclosures made from foil, manufacturers generally use RF gaskets to minimize leakage of large apertures. For example, ventilation panels are generally provided with conductive springs to maintain contact across the seams. Depending upon the seam design used, gaskets will eliminate the need for fasteners or at least will permit them to be spaced further apart.三个基本层的设计有三种基本缝设计;孤立缝(不是一个缝,只是对接),seam压缩和剪切缝。这些是如图4所示。孤立的seam weight-sensitive应用程序中很受欢迎,因为没有重叠的表面,增加材料的内容。同时,屏蔽材料不需要处理垫片压缩力和很瘦的事实,可以使用箔。这个静态(固定)配置轻量级的航天器和卫星应用中经常使用。显然,任何射频密封方法必须在屏蔽材料之间的桥梁。一次性应用程序(不是定期打开),导电带通常是用来密封接缝。seam的应用程序可能会打开,通常由春天的手指桥接配置的差距,双方可以运用部队,或由弹性体,配置适合的差距。如果使用了弹性体,屏蔽材料的厚度必须适当处理边缘加载不屈曲。 One approach to circumventing this problem is to increase the material thickness at the foil edge. The material that is added to increase the edge strength does not have to be conductive. The compression seam is the most popular in situations where there were no initial plans for EMC protection for the enclosure. At the last minute when everything is failing, it is possible to convert an unshielded box with a simple compression seam into a shielded enclosure by adding an RF gasket and more fasteners to the design. This configuration also creates a static joint. In this application, panels overlap the perimeter of the apertures and can be sealed using any type of gasket material. Since the gasket material compression forces are normal to the panel, uniformly spaced threaded fasteners or clamps must be used around the perimeter to maintain the RF seal. The thinner the shield material, the greater the number of fasteners required to assure intimate contact along the seam. As enclosure configurations become smaller and as the choice of complex gaskets becomes quite limited, form-in-place and printed gaskets are used. The shear seam is the only dynamic configuration and differs considerably from the other two types. This type of joint is constructed in several different configurations, viz., pan-edge, knife-edge, modified knife-edge, and/or longitudinal. These designs align the mechanical forces parallel to the panel surfaces and can thus eliminate the need for fasteners. Because of their lower compression forces, metal-finger gasket materials are normally used for this application. It is, however, possible (with some design modification to minimize shear loading) to use fabric-over-foam materials. Besides affording the highest shielding effectiveness over the widest frequency range, shear seam arrangements using metal RF gaskets offer the advantage of being self-cleaning. Because of the considerable difficulty in retrofitting when using shear seams, this design configuration should be determined at the beginning of the project. When this is done, the shear configuration is lower in overall cost than the compression configuration because of the savings in fasteners and the labor to install them.引用:1。詹姆斯·g·Sketoe。“为圆形光阑孔径屏蔽效能:实验数据。“1988项。Robar行业:,西肯肖霍肯PA 2。克里斯托费尔j . Bouwkamp。“通过圆孔衍射的理论和数值处理。“IEEE天线和传播,AP-18卷,2号。1970年3月。3所示。唐纳德·r . j .白色和米歇尔Mardiguian。“电磁屏蔽。“EMC手册,卷。3。 ICT 1988: Gainesville, VA. 4. Wolfgang A. Bereuter and David C. Chang. “Shielding Effectiveness of Metallic Honeycombs.” IEEE Transactions on EMC, Vol. EMC-24, No. 1. February 1982.罗恩·布鲁尔副总裁,EMC /技术服务,是一个NARTE-certified EMC / ESD工程师,他花了超过30年的EMC / ESD /风暴工程领域。他EMC的经验包括项目级别的设计、开发和评价的许多不同类型的电子系统各种FCC /欧盟/ CISPR和军事标准。罗恩被评为杰出讲师由IEEE EMC的社会。他仍获得了工程学和物理学学位从密歇根大学安娜堡分校。罗恩可以达到Laird技术,(570)424 - 8510或访问www.lairdtech.com。