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Letter to the Editor: Comparing Test Methods for EMI Gaskets

To the Editor:

Regarding the article “Comparing Test Methods for EMI Gaskets” by George M. Kunkel (In Compliance Magazine, January 2016), both the text and the caption to Figure 2 mention shielding effectiveness tests performed on “newspaper”??

Figure 2 states that measurements on “newspaper” have varied between 100dB and 60dB, and the text says that this shows that the test method used can give results which are inflated by 40dB.

However, no-one would expect newspaper to have any shielding effectiveness at all (i.e. 0dB), so how can the figure of 60dB in Figure 2 be real?  

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Voltage Standing Wave Ratio results from an impedance mismatch between a source (an amplifier) and a load (test application). This mismatch can influence the performance of the source.

If it was actually newspaper that was tested, I would have expected the author to make some comment about this odd choice of material, and its very odd results (60-100dB instead of 0dB), but he is silent on this point.

So I am left wondering if the word “newspaper” in this article is due to some kind of transposition error or something.

Either way, this mention of newspaper makes the article very confusing indeed. As I am surely not the only person who will be so confused, may I suggest that some sort of explanation is published in the next edition?

Keith Armstrong
Cherry Clough

The Author, George Kunkel, Replies:

The current method of testing EMI gaskets contained in MIL-DTL-83528 has been shown to provide inflated levels of shielding more than 40 dB higher than what can be obtained in actual applications. This inaccuracy is not by chance.

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When this method of test was first proposed in MIL-G-83528, I was chairman of the TC-4 “Technical Committee on Interference Control” of the IEEE’s EMC Society. Our Committee prepared a 14-page critique of the shielding effectiveness (SE) test contained in the document. The critique was sent to the custodian of the document in an effort to revise the SE test, but the idea of a revision was rejected by the custodian. 

We then performed a test on a sheet of newspaper at 2 GHz, which it showed 60, 75 and 93 dB of shielding. The purpose of performing the test with newspaper (which was presented at an EMC Symposium during the 1970s) was to illustrate that the SE test was highly flawed.  (A piece of newspaper should not provide 60-93dB of shielding.)

For that reason, the use of word “newspaper” in our article was in fact intentional. Our point is that newspaper is ludicrous to use and test for shielding, and that a test that shows newspaper rated at 60dB is obviously not a useful or accurate test. And yet it’s been accepted as the industry standard by many.

An alternative method of testing EMI gaskets is transfer impedance testing as described in SAE, ARP-1705.  Transfer impedance testing has proven to be accurate to +/- 2 dB over the frequency range of 10 kHz to 18 GHz. The method of test can: 1) provide a level of shielding of a gasket against any structural material used by industry; 2) test the ability of a gasketed joint to survive the environmental effects (such as moisture and salt fog) to which a system can be subjected; and 3) provide a level of field strength (in terms of E and H fields) at any distance from the gasketed joint for an EM wave impinged on a shielded enclosure. 

See these two articles:
(1) A “historical paper” illustrating that the EMI gasket industry has used the “Shielding Effectiveness” test method to provide the highest level of shielding for their products; and
(2) the results of an analysis that illustrates that the SE test contained in MIL-DTL-83528 can provide inflated level so shielding up to 70 dB at any given frequency.

George Kunkel
Chairman “Technical Committee on Interference Control,” EMC Society of the IEEE (1969 – 1987)

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