EMC Basics #11: An introduction to EMC shielding

[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.]

Time to shift gears, and look at the mechanical side of EMC — or more correctly, the electromechanical side of EMC. In this next mini-series, we'll look at various aspects of shielding, and how it works. It is not enough to just hang metal — you need to understand what you are doing, and why.

The primary purpose of a shield is to block electromagnetic radiation. This includes both radiated emissions and radiated susceptibility. In fact, most of the time a shield behaves in a reciprocal manner, and what works for one direction works equally well for the other. The exceptions are subtle, and will be ignored for now.

Shields can be applied at different levels. The most common for electronic equipment is at the "box" level, but shielding can also be employed at the component, board, or even systems level.  (In the latter case, think of shielded rooms.)  In fact, multiple levels of shielding are quite common.  You don't need to depend on just one shield for all your EMI protection.

Shielding performance is traditionally defined as "shielding effectiveness" (SE). This is the ratio of the field level before the shield is in place, divided by the field level after the shield is in place. It is customary to express this parameter in deciBels. The number should always be zero (no shielding) or positive. (If negative, you must be creating energy. Quick — patent it!)  

As with many EMC issues, there is a lot of duality with shielding:

•Two modes - reflection and absorption

•Two design issues - materials and mechanical

•Two field concerns - near field and far field

•Two frequency concerns - low frequency and high frequency

•Two impedance concerns - low (magnetic) and high (electric)

With all these variables, it is no wonder one shield design does not work for all cases. 

All of the above leads to thinking of shielding in three regimes: magnetic, electric, and electromagnetic. Figure 1 is a curve from a military design handbook showing the SE of copper. 

Notice the two modes and three regimes.  Don't panic: we'll look at this in more detail to help you decode the mysteries of shielding.

Figure 1: Typical shielding curves for copper

As you can see, there are a number of things to consider when designing an EMC shield, both electrical and mechanical. If you need shielding, it is not enough to just throw it over the wall to the mechanical engineers. You need to be involved in the design decisions, too.

We'll explore all of these topics in more detail in future posts. We'll also augment these with practical design guidelines along with our favorite shielding "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/.