Conformity Magazine- May 2008 - (Page 50) high shielding effectiveness at RF frequencies, although their magnetic shielding effectiveness generally falls off at low frequencies (only an issue in special situations). Lets examine at the basic mechanisms underlying reflection and transmission. When electric and magnetic fields impinge on a conductor, there will be some reflection and some transmission at the boundary. Reflection occurs at a boundary where there is a difference in the conductivity, permittivity, and/or permeability of two neighboring materials. The greater this difference is, the greater the reflection. For plane waves, and metallic levels of conductivity, reflection will provide attenuation of the transmitted signal of 100 dB at any frequency of interest, although it does decrease with frequency. Absorption occurs in conductors as fields attempt to travel through them. Sinusoidal electric and magnetic fields are attenuated exponentially with distance in a conductor. This is commonly called the “skin effect” phenomenon. Alternating fields travel primarily at the surface of conductors, in a skin layer. The is a characteristic skin depth in conductors given by: Skin depth = where meters, (1/e), or 8.78 dB for each skin depth into the material. We’ll discuss the effect of skin depth in more detail below, under the heading of “Thickness.” The attenuation provided by the mechanism of absorption increases with frequency. Both reflection and absorption are at work for most applications. Conductivity The first factor that affects the shield is the conductivity of its surface. This directly reflects the attenuation due to reflection, and also affects the amount due to absorption. There are many possibilities for the shield material, but for all of them, the conductivity will generally be great enough to provide a high degree of reflection. Practical shields always involve metal in some form, but the shielding material may be “thick” (i.e., multiple skin depths at the frequencies of interest) or thin. The metal itself may vary substantially in form and conductivity. The metal may be pure or plated; it may be a solid thick enough to be structural or it may be a coating or deposition on a non-conductive plastic, or it may come in the form of a paint. Conductive paints are usually applied so they are several thousandths of an inch thick, or about .01 cm. The usual figure of merit one looks at with conductive paints is the “surface resistivity” (or if you like, its reciprocal, the surface conductivity), which has the units of “ohms per square.” This is the resistance that will be seen from one edge to the other of a “square” section of painted surface, which, it happens, will be the same regardless of the size of the square (Figure 2) shows why this is so. Practical experience has shown that the rough dividing line between paints that are effective as shields and those which are not as reliable occurs in the vicinity of 1 to at most 2 ohms per square. Lower values work better; higher ones are problematic. Since paints are “thin” coatings, the mechanism by which they shield is primarily reflection. The greater the difference between the wave impedance in air (377 ohms in the far field) and the value in the conductor, the greater the shielding provided by the mechanism of reflection. Typically, nickel paints are in the 0.75 to 2 ohms per square range; copper paints (either alone, or alloyed with other metals such as nickel or small percentages of silver) are about 4 or 5 times more conductive. Interestingly, it turns out that these paints, conductive as they are, are approximately two orders of magnitude less conductive than a pure metal layer of the same thickness as the applied paint. This is at first glance a bit surprising, because the volume percentage of metal in these paints is quite large (of the order of 50%). Apparently, the conductive particles contact each other frequently enough to give a decently low resistance, but particle-to-particle contact is far less intimate than in a solid, because of the insulating effect of the organic compounds which hold everything together once the paint w = angular frequency (or 2pf) m = permeability s = conductivity Figure 1 shows the values for some common materials as a function of frequency. Fields are attenuated by a factor of Skin depth is given by the formulae below. Using values of permeability relative to free space, and conductivity relative to that of copper makes it easy to see the dimensions. Absorption attenuation falls off as e-(x/d), or 8.78 dB per skin depth Skin depth for some common materials is given. For magnetic materials, since permeability declines with frequency, the skin depth at high frequencies is uncertain. Frequency Copper (in.) 100 Hz 100 Khz 100 MHz 1000 MHz .260 .008 .00026 .00008 Aluminum (in.) .333 .011 .0003 .0001 Steel (in.) .027 .0008 Depends Depends Mu-metal (in.) .011 Figure 1 50 Conformity mAy 2008
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