More Letters on Shields and Grounds

DN Staff

February 26, 2010

4 Min Read
More Letters on Shields and Grounds

Recent Tips from Titus columns — “An Update on Shielding” (11-2009) and “Look Deeper at Shielding”– in Design News magazine prompted e-mails from readers. I ran four letters in the March 2010 Tips column in Design News and include some others below. For the original columns, see the links at the end.

1. I think the reasoning behind the 1/20-wavelength rule of thumb is this: A dipole antenna — a tuned circuit — serves as a model of the shield. Tuned circuits have a roll off of roughly -40 db per decade. At 1/20 the wavelength of the highest frequency, this is about -44 db, or 99 percent attenuation. That seems good enough to easily remember.
— (Name withheld)

2. In the November issue of Design News, Jon Titus wrote, “The twisted-pair cable, A, has a 100-percent aluminum-foil shield and separate ‘drain’ wires that connect to the shield and simplify a ground connection. These separate wires have a lower resistance than the foil alone.”

The shield is not “100-percent aluminum foil.” Aluminum foil would be too delicate and would tear if the cable was bent. The “foil” is actually Mylar plastic coated with a thin layer of aluminum on one side. The drain wire serves two purposes.
First, if the metallization cracks and opens electrically, the wire provides a continuous path that bridges the crack. Of course, the shield no longer offers 360-degrees coverage at that point and high-frequency performance is compromised.
Second, the drain wire provides a way for a connector to make contact with the thin aluminum layer. At the connector, most people undermine the effectiveness of a foil shield. Grounding through the drain wire seriously compromises the high-frequency effectiveness of the shield because the thin drain wire becomes inductive at high frequencies. Therefore, the length of the drain wire (at the connector) must be kept as short as possible.

A metal shield blocks an electric field. Even an ungrounded shield will do that. A typical shield on a shielded cable is very ineffective at blocking magnetic fields. –Chuck Gollnick, Lead Electrical Engineer, Beaverton, OR.

3. I found the reference, “Grounding and Shielding for Sound and Video,” by Philip Giddings, and thought it might help others like me who are mechanical rather than electrical engineers. Here’s what Giddings said in Section 3.3.2:

“Any opening in the shield will diminish its effectiveness. The most interesting thing about electromagnetic leakage through slots is that the longest dimension on the opening is critical not the total area of the opening. For example, a 10-by-1/10th inch slot will be about 10 times more leaky than a 1-by-1 inch square hole, even though both have the same total area. The general guide line is that slots should be kept shorter than 1/20th of the wavelength of the highest frequency of concern. For example, at 100 MHz the slot should be less than about 6 inches, 300 MHz about 2 inches and 1,000 MHz about 2/3 of an inch. These dimensions will assure at least 20 dB of attenuation at the highest frequency. For 40 dB of attenuation you would need to keep the slots shorter than 1/200 wavelength.” –Howard Kasper, Mechanical Engineer, Carlsbad, CA.

4. Cable shielding remains a mystery to many people and is not well taught in schools. Also, old habits and rules-of-thumb, whether good, bad or inconsequential, seems to dominate many designs.

I design many shielding arrangements; most for the relatively benign environment of 4-20-mA signals where almost anything reasonable will work. In a past life, I did more extensive work in rigorous analog-instrumentation environments. I have collected articles on the shielding and refer to them when in doubt, or when a customer quizzes me about my designs.

First, I like two articles published in Analog Dialog from Analog Devices; “Grounding for Low- and High-Frequency Circuits,” by Paul Brokaw and Jeff Barrow, Vol. 23, No. 3, 1989 and “Shielding and Guarding,” by Alan Rich, Vol. 17, No. 1, 1983.

Second, I like the book “Grounding and Shielding Techniques in Instrumentation” by Ralph Morrison. This is a theory-heavy volume that also condenses bits of wisdom in the form of “rules” that engineers can follow without getting deep into theory. The rules generally correlate directly with those listed in the Analog Dialogue articles noted above and include some refinements. –Paul Schmidt, Senior Electrical Engineer,
Waukegan, IL.

Here are the links to the published columns:

“An Update on Shielding,” www.designnews.com/article/388473-An_Update_on_Shielding.php.

“Look Deeper at Shielding,” www.designnews.com/article/439434-Look_Deeper_at_Shielding.php.

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