Electromagnetically shielded rooms, also called Faraday cages or screen rooms, are seeing widespread use today. But while many companies are buying their first shielded rooms, the basic technology has been around for more than 40 years. Originally limited to military and electromagnetic compatibility (EMC) testing, shielded rooms now have many commercial uses.
Generally, the shielded enclosure should keep ambient RF signals out and create a quiet environment. It also should keep signals in, such as when generating high-powered RF signals to test the susceptibility (immunity) of a product.
Standards
Because of its proven usefulness and easy availability as a free, unclassified document, MIL-STD-285, Method of Attenuation Measurements for Electromagnetic Enclosure Shielding for Electronic Test Purposes, has become the de facto world standard for testing the effectiveness of shielded rooms. The original version was written in 1956. Today, most users add higher test frequencies, and some change other details to compensate for the technological advances of the past 45 years.
The U.S. government has prepared other standards for shielding effectiveness testing. However, most are not available to the public.
The Institute of Electrical and Electronics Engineers (IEEE) produced IEEE 299-1991, Standard for Measuring the Effectiveness of Electromagnetic Shielded Enclosures. This document is not widely accepted and needs modification in many areas.
The Canadian government released an excellent new standard, ITSG-02, in December 1999. This practical, well-developed document obviously was written by people with a great deal of experience in testing shielded rooms. With some improvements, ITSG-02 eventually could replace MIL-STD-285 as the world standard for testing and certifying shielded rooms. This unclassified document isn’t copyrighted and is readily available at no charge.1
State of the Industry
During the Cold War, the shielded-room industry enjoyed decades of closely regulated government business. Demand was steady, and quality was kept high under the close supervision of experienced government engineers.
This demand dropped sharply when the Berlin Wall came down. Many shielded-room manufacturers went out of business or into other types of construction.
About five years ago, the industry began to revive because of several developments:
- The rigorous new EMC requirements imposed on consumer products by the European Community.
- The advent of MRI technology, which is susceptible to RF interference.
- The growth of wireless equipment, which must be tested in quiet electromagnetic environments.
However, many EMC experts left the trade before the recent boom began. And with no more government supervision, the industry, in general, has become somewhat chaotic. Consider this:
- There are no special licensing programs for manufacturers of shielded rooms. Any construction firm can build one, even though the techniques are quite different from those used in other types of construction.
While any competent construction firm can provide a solid, attractive room, it could be full of RF leaks. These leaks usually are very hard to find and fix. It is a costly proposition, and often you pay more and get less performance than if the job had been done by experienced professionals. - Unlike most trades, there are very few training or apprenticeship programs for the craftsmen that erect shielded rooms.
- There is no independent certification program for technicians who validate shielded rooms. Most test personnel work directly or indirectly for the builder, which could make their findings suspect.
Shielded Rooms and Shielded Enclosures
Often the terms shielded room and shielded enclosure are used interchangeably. A shielded room is a shielded enclosure, but not all shielded enclosures are rooms.
A shielded enclosure can be anything from a tiny shielded can on a PCB to a huge chamber suitable for testing a satellite or an airplane. An enclosure should not be called a shielded room unless it is at least as big as a closet. You must be able to walk into the shielded room.
Types of Construction
The most commonly used shielded rooms are the bolt-together types, typically using 4¢ × 8¢ panels. Each panel is a sheet of particleboard sandwiched between two sheets of galvanized steel. An entire room comes in a kit and can be assembled completely from the inside. This is a mature, proven technology.
Other techniques include all-welded, prefabricated assemblies and rooms made of foil on sturdy frames. You even can make a shielded room using conductive paint or wallpaper. Generally, these are lighter in weight than the prefabricated, steel-lined enclosures but less effective, less reliable, and about the same price.
Still another variation is the shielded tent. This enclosure provides 60-dB shielding effectiveness. It can be broken down into a small, lightweight package for transporting, then reassembled and retested for use. For some applications, these represent the wave of the future.
How Much Shielding Is Enough?
With all the choices, picking the right room can be a bewildering task. The easy approach is a bolt-together room that provides at least 80-dB shielding effectiveness. However, it is better to conduct a survey, measuring the ambient RF signals of the rooms you are considering and comparing them with your requirements.
Look carefully at the typical weak spot—the door. The gaskets must be in good condition and not subject to damage in normal use. For example, the threshold can be rendered less effective by an accumulation of dirt.
Basic Testing for Compliance
While you may hire a consultant for verifying proper shielding, there are some basic tests that you can do. First, set up a transmitter and receiver at a given distance from each other and take a reference reading of the received signal strength. Then move the receiver inside the room, place it at the same distance and antenna orientation as before, close the door, and note the difference in the signal level.
You should cover the frequency range of concern, possibly as great as 1 kHz to 18 GHz. Expect at least 80-dB attenuation at 400 MHz and 1 GHz.
Use caution in this test. Generating these signals may violate the law and be harmful to people with prosthetic devices such as pacemakers.
Selecting a Consultant
How do you find an independent consultant to give advice on purchase, installation, and test? It isn’t easy. You can’t locate someone with a master’s or doctorate degree in EMC technology because this is a derivative field, one that isn’t taught as part of the fundamental body of engineering knowledge. No state licenses EMC engineers, even though all states license the broad category of electrical engineering. Consequently, it is not easy to discriminate among those offering their services.
However, there is an organization that can evaluate the qualifications of would-be experts in shielded-room matters. According to Russell Carstensen, executive director of the National Association of Radio and Telecommunications Engineers (NARTE®), “Our organization conducts certification of professional credentials based on education, work experience, peer endorsement, and academic examination. We verify that a candidate for certification has the understanding of EMC principles and a demonstrated track record of professional performance. This is the only way to determine who has measured up to objective technical standards in the field.”
Getting Smart
If you are about to buy or build your first shielded room, it is a good idea to learn a bit about the subject first. The Armed Forces Communications and Electronics Association (AFCEA) offers an economical series of courses on this subject.2 The first of the series, Shielded Enclosures, is a one-day program and all you’ll need unless you are planning to build or test the room yourself. If you don’t have the time to attend a course, read some of the tutorial books from NARTE or the IEEE Press.3, 4
Choosing a Builder
When choosing a builder for a room, the most important consideration is the experience of the project manager. This responsibility doesn’t require impressive qualifications, but rather a great deal of experience in building properly functioning shielded rooms. It is wise to verify this person’s credentials by interviewing previous customers. Don’t select the builder based on the size of the company, the slickest sales pitch, or the lowest bid.
Additional Reading
- Gnecco, L.T., The Shielded Enclosure Handbook, TEMPEST INC., 1999.
- Hemming, L., The Architectural Electromagnetic Shielding Handbook, IEEE Press, 1992.
- Gnecco, L.T., The Design of Shielded Enclosures, Butterworth-Heinemann, 2000.
- Gnecco, L.T., Problems and Solutions in Wireless Communications and Electromagnetic Compatibility, TEMPEST INC., 1999.
References
- Canadian Document ITSG-02, [email protected]
- AFCEA Training Courses, www.afcea.org/education/schedule.html
- NARTE books on screen rooms, www.narte.org/emc.html#study
- IEEE books on screen rooms, www.ieee.org/organizations/pubs/press/
About the Author
Lou Gnecco, president of TEMPEST INC. for the past 16 years, is a certified EMC engineer and a certified TEMPEST professional, Level II. His professional experience includes five years with the National Aeronautics and Space Administration and 14 years with the Department of Defense. Mr. Gnecco has a B.S.E.E. from Manhattan College and an M.S.E.E. from George Washington University. TEMPEST INC., 112 Elden St., Herndon, VA 20170, 703-836-7378.
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April 2001