This article assumes the reader already has a basic understanding of how electromagnetic compatibility (EMC) shielding works. If you would like a refresher on EMC shielding, please see references 1 and 2 which describe how EMC shielding works in basic terms.
If you’re an electrical engineer and consider proper EMC shielding a mechanical only issue, please reconsider your position and keep reading. Proper EMC shielding is both an electrical and mechanical design issue. Thoughtful coordination between the two disciplines is key to successful EMC shielding implementation on most product development projects.
The remainder of this article is based on a situation we have all experienced. It’s the situation in which we think we can rely on a certain level of dB in shielding effectiveness (SE) for a product, assuming there is no problem and we had plenty of SE, only to find out later (usually too late) that when the shielding is eventually installed in the end-use product/application and tested, we actually obtain much less SE than required. Our product fails compliance testing and we are sent back to the drawing board, scurrying with vigor to develop a solution fast, and with the least cost possible.
This article covers some of the most common reasons why our first attempts at proper shielding as described in the situation above were all in vain. The problem is usually that the EMC shielding design incorporated the weaknesses described below from initial inception and the SE was already defeated by design.
Shielding Effectiveness from 30 to 1000 MHz
The focus for this article is also in the frequency range of 30 to 1000 MHz as most shielding difficulties are in this RF range. This is where most of the requirements for electrical/electronic products reside due to constraints imposed by FCC/EU radiated-emission and EU immunity standards.
Predominate Defeaters of RF shielding
Most high-frequency shielding problems are caused by openings in material, not the material itself. The predominant two defeaters of RF shielding are slots and penetrations.
In the example situation noted above, the EMC shielding may not have provided enough SE because the slots in shielding were too long and therefore could easily leak RF energy into and out of them. Remember that it’s the longest dimension of the slot that is the critical factor, not its area. In general, to prevent a slot from leaking and destroying SE, we should have kept the slot length less than about one-twentieth of a wavelength at the highest frequency of concern. The highest frequency of concern for the situation described above is 1000 MHz, which means a slot length of no more than about 1.5 cm or around 2/3 in.
Another reason why the SE for the example situation may have not been good enough is because slots in the shield were rendered useless by the application of non-conductive paint or covers, or possibly by poor-fitting panels or doors. These effects may have gone unnoticed as they are sometimes harder to locate than the more visible issue of the slot being too long. From now on, all new shielding and enclosure designs will be checked to ensure these issues are avoided.
The EMC shielding in our situation may have been defeated by something as simple as an open and unenergized insulated wire running through an opening in the shielding. The wire could have carried RF energy into and out of the shielding instead of terminating to it. As an experiment, try using your cellphone inside of an EMC chamber with the door closed. Caution: Make sure the EMI receiver and other test equipment are turned off before doing this to prevent any damage to them from the use of the cellphone. See if you can make a call with everything buttoned up with a tight RF seal. Next, run a simple insulated and unfiltered wire through the pipe penetration located on the side of the chamber. Close the door and recheck your cellphone signal. You will be surprised how much of a signal you will now be able to get with the insulated wire running through the penetration even though the door to the chamber is closed.
The shielding may have also been compromised due to other penetrations through shielding necessary for proper product functionality. These could include intended conductors, such as signal/power lines or unterminated cable shields. In these instances, we should have added proper EMI filtering of these lines that would have helped reduce their impact on defeating the shielding.
For more information on this important subject, please see the following:
- In Compliance Magazine. (2018, August). What Every Electronics Engineer Needs to Know About: Shielding. Retrieved from https://incompliancemag.com/article/what-every-electronics-engineer-needs-to-know-about-shielding/
- Gerke, D., Kimmel B., The Designer’s Guide to Electromagnetic Compatibility, EDN, 2005.
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