EMC Basics #13: EMC shielding--destroying a shield

[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.]

There are two ways to destroy a high-frequency shield: seams and penetrations. As discussed in the previous article (EMC Basics #12), even thin conductive materials work well for frequencies above about 10 kHz. Thus, the weak points are mechanical rather than materials.

Building a high-frequency shield is like building a wooden water tank. Once the planks are thick enough, the leaks occur at seams, joints, penetrations, and even knotholes. And even a small hole can be a problem : drill a ¼ inch hole in the bottom of the tank, and eventually all the water leaks out.

Incidentally, the “planks” don’t need to be thick for high-frequency EMC shielding, as long as they are conductive. Obviously, wood doesn't work for EMC, but aluminum foil is very effective, along with surface treatments like conductive paint or plating. Remember, however, you still may need the  “thick conductive planks” for low-frequency magnetic field shielding (60 Hz and harmonics).

Before we go further, we need to look at some simple physics. Although the water-tank analogy is useful, it falls apart in several ways. Here are a couple of key points:

•For seams, the longest dimension is critical -- NOT the area. Unlike water, a six-inch seam will leak the same as a six-inch hole under worst-case conditions. The difference is that the seam will be highly polarized, while the hole will not. When designing an EMC enclosure, we need to be pessimistic. After all, Murphy and his law will make sure that the worst case will occur.

•For penetrations, the depth of penetration is critical, NOT the hole size. If a  wire, cable, or even a pipe extends beyond the EMC shield and is NOT shorted to the shield, the extensions act like antennas connected by a coaxial cable. For example, carrying wires or cables through a hole to a connector on the circuit board can completely destroy a shield at high frequencies. 

A good rule of thumb for seams and penetrations is the “1/20 wavelength rule.” Antenna designers often use this guideline as a practical limit when making small antennas. You don’t need a half wavelength (or even a quarter wavelength) to support electromagnetic radiation – 1/20 of a wavelength will still do a credible job.

As EMC designers, we’re trying to do just the opposite – that is, NOT  design antennas into our shields. Yet, that is what happens. The seams look like slot antennas, and the penetrations look like monopole antennas. Both can radiate (leak) a surprising amount of energy at high frequencies.

Most EMC engineers use a 1/20 of a wavelength as a starting point, but even that only provides about 20 dB (10×) reduction. If you need 40 dB (100×) this reduces to 1/200 wavelength, and  60 dB (1000×) reduces to 1/2000 wavelength.

You can quickly calculate physical dimensions using this formula, based on the speed of light in free space:

Frequency (MHz) × Wavelength (meters) = 300

•For example, at 100 MHz, a wavelength is 3 meters, so a 15-cm seam or penetration (about six inches) provides 20 dB of shielding. If you need 40 dB, that reduces to 1.5 cm, and if you need 60 dB, that further reduces to 1.5 mm.

•It’s even worse at 1 GHz, where a wavelength is 30 cm. A seam or penetration of 1.5 cm (less than an inch) only provides 20 dB of shielding. If you need 40 dB, that reduces to 1.5 mm, and 60 dB it is only 0.15 mm. No wonder we need EMI gaskets (or even welded seams) and bulkhead connectors at the higher frequencies!

We’ll look at how to plug those leaks in a future post of this series. In the meantime, the 1/20^th wavelength rule is a good place to start. After all, if you can’t get a 10× reduction, why even bother with shielding in the first place?

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/.