[Editor's note: we are pleased to continue our series on the vital and sometimes unappreciated topic of electromagnetic compatibility (EMC), presented by well-known expert Daryl Gerke of Kimmel Gerke Associates. Note that there are links to all previous entries here.]
The first shielding decision is usually about materials. Do we need thick steel or mu-metal, or will a thin shield or even conductive paint suffice? Well, it depends...
To better understand this issue, we'll delve into basic shielding theory. Don't panic ó we won't be deriving Maxwell's famous equations or bogging you down in electromagnetic field theory. Rather, we'll use a simple theory developed in the 1930's by Dr. Sergei Shelkunoff that still serves us very well almost a century later.
If you look at the shielding diagram in the previous post, you can see that prior to Shelkunoff, it must have been very difficult to figure out what was going on. Did shielding increase or decrease with frequency? Was it linear or exponential? What was the rate of change? The answer, common in the EMI world, was "It depends..."
Shelkunoff proposed a simple transmission line model for shielding ó specifically a lossy transmission line. This resulted in two major mechanisms, reflection (R) and absorption (A). He also added a fudge factor for reflections through a thin shield that he dubbed B.
The resulting top-level equation was rather elegant:
SE (dB) = R(dB) + A (dB) + B
The mechanisms are illustrated in Figure 1.
Figure 1: Basic shielding mechanisms modeled as transmission-line effects.
Incidentally, since B is relatively small for most EMI issues, most of us just ignore it. (It can be important, however, for very thin shields or at optical frequencies.) As such, we'll focus on the two remaining shielding mechanisms, R and A.
Reflection: This is a surface mechanism, and is the result of the mismatch (transmission line theory) of the "barrier impedance" and the "wave impedance." The former is simply the surface impedance, given in "ohms/square", while the latter is the ratio of the magnitudes of the electric and magnetic fields.
In free space, the wave impedance is 377 ?, but at very low frequencies (such as 50/60 Hz) the wave impedance may be drastically altered by the circuit impedance.
Since the barrier impedance for metals and metallic coating is often measured in milliohms, you can see we have a huge mismatch at higher frequencies. As a result, reflection is the primary mechanism for shielding at radio frequencies (RF) above 10 kHz. Even thin coatings like conductive paints can provide 60-80 dB or more of shielding.
Absorption: This is a volume mechanism, and is the result of loss through the shield (lossy transmission line theory), and is the result of "skin depths." The loss is exponential. One skin depth results in 8.68 dB (one neper) of loss, two skin depths in 17.4 dB of loss, etc. Since we usually don't need absorption for RF frequencies, any added absorption is a bonus.
At power frequencies with high currents (low-impedance fields), the reflection is minimal so you need absorption. Furthermore, skin depths are hard to come by at low frequency. At 60 Hz, you need at least 3-4 inches of aluminum to start to have even a small effect. So what to do?
Well, you can boost the skin depth by permeability. The improvement is proportional to the square of the relative permeability. Thus, 0.1 inch/2.5mm of steel ( Ķr= 1000) gives the same absorption as about 3 inches/75mm of aluminum. This is why we use steel (or other permeable materials) for shields around power supplies or around devices that are sensitive to power-line magnetic fields.
The bottom line: the two mechanisms often drive our choice of shielding materials. At RF frequencies (above 10 kHz) thin conductive coatings are usually fine. For power frequencies (50/60/400 Hz) with high currents, thick permeable materials are often required.
In the next post, we'll start to look at the mechanical issues of shielding, such as how seams and other discontinuities affect RF shielding. These are the common limitations of RF shielding. We'll also share some of our favorite "rules of thumb."
Also relevant to this topic:
Debugging: The 9 Indispensible Rules for Finding Even the Most Elusive Software and Hardware Problems (Chapter 5, Part 3 of 3) (and see its preceding sections, which are linked within)
About the author
Daryl Gerke, an EMI/EMC consultant since 1987, along with business partner Bill Kimmel, focuses on design and troubleshooting (not test and regulations). He and Kimmel have been chasing EMI problems for over 80 years (combined, of course.) He is a published author and columnist, and their EDN Designer's Guide to EMC (1994) is still relevant and in demand. He can be reached via http://www.emiguru.com or his other blog at http://www.jumptoconsulting.com/.