Shielding Effectiveness
屏蔽效率
Most shielding effectiveness data given in Table 7 of the Extrusion Product Guide is based on a MILDTL test method, with a 24 in. x 24 in. aperture in a rigid enclosure wall and about 100 psi on the gasket. It is a valid and useful way of comparing various gasket materials, but does not reflect the shielding effectiveness one can expect at seams of typical enclosures. CHO-TM-TP08 is a modified version of the MIL test that provides typical values achieved in actual applications. Since many factors will affect the actual shielding effectiveness of an enclosure seam (flange design, stiffness, flatness, surface resistivity, fastener spacing, enclosure dimensions, closure force, etc.), the only way to determine shielding effectiveness for real enclosures is to test them.
压出制品手册中表7中所给出的屏蔽效率数据是基于MILDTL试验方法，采用刚性外壳壁内一个24x24英寸的孔，垫片上压力为大约100磅/平方英寸。这种方法可以有效且有用的对各种垫片材料进行对比，但是不能反映人们对典型外壳接缝处所预期的屏蔽效率。CHO-TM-TP08是MIL试验的改进版，它可提供在实际应用中获得的典型数值。由于许多因素都会对外壳接缝的实际屏蔽效率产生影响（法兰设计、刚度、平面度、表面电阻系数、扣件间距、外壳尺寸、闭合力等），所以要想确定现实外壳的屏蔽效率，唯一的方法就是对其进行试验。
Figures 9 and 10 provide data on shielding effectiveness for actual enclosures. The data in Figure 9 shows the difference in attenuation between a shelter door closed with no gasket and the same door closed against a CHO-SEAL 1215 hollow D-strip gasket. Instead of single data points at each frequency tested, a range of data is shown for each frequency, representing the worst and best readings measured at many points around the door. Figure 10 shows the effects of closure force on shielding effectiveness of an enclosure tested at high frequencies (1-40 GHz) using CHO-SEAL 1215 solid D-strip gaskets.
图9和图10提供了实际外壳的屏蔽效率数据。图9中的数据给出了不带垫片的关闭的掩体门和带CHO-SEAL 1215空心D形条垫片的关闭的相同门之间的衰减差异。不是在每个试验频率下测出单个数据点，而是每个频率都显示一个数据范围，其代表在门周围许多点所测量的最差读数和最好读数。图10给出了在高频（1-40 GHz）下对带实心D形条垫片的外壳进行试验，闭合力对其屏蔽效率的影响。
华译网上海翻译公司曾经翻译过大量有关屏蔽效率资料文件，Beijing Chinese Subtitling Translation Service Agency has translated many technical documents about Shielding Effectiveness.
Figure 9. Shielding Effectiveness of a Shelter Door Gasket (14 kHz to 10 GHz)
图9：掩体门垫片的屏蔽效率（14 kHz 至10 GHz）
Figure 10. Effect of Closure Force on Shielding Effectiveness (1 GHz to 40 GHz)
图10：闭合力对屏蔽效率的影响（1 GHz至40 GHz）
In order to establish reasonable upper limits on gasket resistivity, it is necessary to understand the relationship between flange interface resistance and EMI leakage through the flange. Figure 11 presents this relationship for an aluminum enclosure 3 in. x 3 in. x 4 in. deep, measured at 700 MHz. Die-cut gaskets 0.144 in. wide by 0.062 in. thick, in a wide range of resistivities, were clamped between the gold-plated flanges of this enclosure. Simultaneous measurements of flange interface resistance (all attributable to the gaskets) versus RF leakage through the seam produced a classic S-shaped curve. For the gasket configuration used in this test, the dramatic change in shielding effectiveness occurs between gasket volume resistivities of 0.01 and 0.4 ohm-cm. Since real enclosures do not have gold-plated flanges, but rather have surface finishes (such as MIL-DTL-5541F, Type I, Class 3 chromate conversion coatings) which also increase in resistance over time, it is recommended that gasket volume resistivity be specified at 0.01 ohm-cm max. for the life of the equipment.
为了对垫片电阻系数建立合理的上限值，有必要理解法兰界面抗阻和法兰电磁干扰泄漏之间的关系。图11表示的是在700 MHz下测量得到的3x3x4英寸铝制外壳的上述关系。 0.144英寸宽、0.062英寸厚，大范围电阻系数的冲切垫片被夹在该外壳的镀金法兰之间。同时测量法兰界面抗阻（全部可归因于垫片）与从接缝处的射频泄漏之间的函数关系，从而生成一个标准的S形曲线。对于本试验中所采用的垫片结构，当垫片的体积电阻系数由0.01到0.4欧姆厘米之间变化时，屏蔽效率会发生显著变化。由于现实外壳没有镀金的法兰，而是对表面进行抛光（例如MIL-DTL-5541F的I型，3级铬酸盐转化涂层），这也会随着时间的推移而引起电阻的增加，为了设备寿命考虑，建议垫片的提及电阻系数规定为最大0.01欧姆厘米。
Figure 11. Interface Resistance vs. Shielding Degradation at a Flange Joint
图11：在法兰接口处的界面抗阻和屏蔽损失之间的函数关系
EMP Survivability
电磁脉冲残存性
In order for an enclosure to continue providing EMI isolation during and after an EMP environment, the conductive gaskets at joints and seams must be capable of carrying EMP-induced current pulses without losing their conductivity. Figure 12 shows the EMP current response of various types of conductive elastomer gaskets. Note that gaskets based on silver-plated-glass fillers (1350) become nonconductive at low levels of EMP current, and should therefore not be used when EMP is a design consideration. Figure 13 is an electron microscope photo which clearly shows the damage mechanism. Silver-plated-copper filled (1215) gaskets have the highest resistance to EMP type currents, showing no loss of conductivity even at 2.5 kA/inch of gasket (peak-to-peak). Pure silver (1224) and silver-plated-aluminum filled (1285) gaskets have less current carrying capability than silver-plated-copper materials, but are generally acceptable for EMP hardened systems (depending on specific EMP threat levels, gasket cross section dimensions, etc.).
为了使外壳能够在电磁脉冲环境期间及之后都能够持续提供电磁干扰隔离，在接头和接缝处的导电垫片必须能够承载电磁脉冲的感应电流，而不会损失其传导率。图12给出了各种类型的导电弹性体垫片的电磁脉冲电流响应。注意，基于镀银玻璃填充剂（1350）的垫片在低水平电磁脉冲电流下会变为不导电，因此当设计考虑电磁干扰时不应使用这种垫片。图13是一张清楚显示损坏机理的电子显微镜照片。镀银铜填充（1215）的垫片具有电磁脉冲型电流的最大阻抗，即使在2.5kA/英寸下（峰－峰），也不会损失传导率。纯银（1224）和镀银铝（1285）填充的垫片的载流能力比镀银铜材料的载流能力小，但是通常能够满足电磁脉冲硬化系统的要求（取决于特定的电磁脉冲威胁等级、垫片横截面尺寸等）。
Figure 12. EMP Current Response of Conductive Elastomer Gaskets
图12：导电弹性体垫片的电磁脉冲电流响应
Vibration Resistance
抗振性
Certain conductive elastomers are electrically stable during aircraft-level vibration environments, while others are not. The key factor which determines vibration resistance is the shape and surface texture of the filler particles. Smooth, spherical fillers (such as those used in silver-plated-glass materials) tend to move apart during vibration, leading to dramatic increases in resistance and loss of shielding effectiveness (although they normally recover their initial properties after the vibration has ended). Rough, less spherical particles resist vibration with very little electrical degradation. Figure 14 shows the effects of vibration on three types of conductive gaskets. Although XXXX silver-plated-copper filled 1215 gasket, with rough, irregular particle agglomerations, exhibits excellent stability during vibration, users of conductive elastomers should be aware that smooth, spherical silver-plated-copper fillers can be almost as unstable as silver-plated-glass fillers.
有些导电弹性体在飞行器级别的振动环境中能够保持电气稳定性，而其他的就不行。决定抗振性的关键因素是填充颗粒的形状和表面结构。光滑、球形的填充剂（例如采用镀银玻璃材料的）在振动过程中趋向于散开运动，从而导致电阻显著增大，而屏蔽效率受到损失（尽管它们在振动结束之后通常能够恢复其初始特性）。粗糙的、非球形的颗粒可抗振，电气损失很小。图14给出了三种类型导电垫片的振动影响。尽管XXXX 镀银铜填充的1215垫片具有粗糙、不规则的颗粒结块，在振动过程中显示出卓越的稳定性，但导电弹性体的使用者应了解光滑的、球形镀银铜填充剂几乎与镀银玻璃填充剂一样不稳定。 |
