All over the world, a growing number of EMC test laboratories are faced with a shortage of testing capacity for radiated immunity and emissions testing. Lab managers must balance available facility space and company budgets with estimated testing demand and revenue. As a consequence, many labs do their own design, construction, and installation of ferrite tile absorbers in shielded enclosures.
For many smaller companies, it simply is not a financial option to contract a turnkey chamber project to one of the major chamber providers. For example, while a 10-m chamber is highly desirable, the $1M to 2M price tag makes that old outdoor site look pretty attractive.
The question is: “Can an economical ANSI-compliant anechoic chamber be built?”. The answer is yes, if the trend in do-it-yourself chamber construction continues.
Entrepreneur Spirit
While some may wither at the thought of such a project, for years, many EMC labs have constructed special items such as antennas, ground planes, screen rooms, and electrical filtering out of financial necessity. Building a chamber via the low-cost approach requires a willingness to improvise using available components and materials, both new and used.
One example of this ingenuity is the recently completed 3-m ferrite-lined chamber located at Hermon Laboratories in northern Israel. It is the only fully welded, ferrite-lined, FCC-filed, 3-m site in Israel built and assembled by its owners. This approach saved an estimated 30% in total cost from construction to final testing.
Planning and Construction
Hermon Laboratories designed the parent building to accommodate a 3-m chamber. The building’s layout provided adequate height for the 6-m H × 7-m W × 10-m L shielded room and 2.5-m × 2-m roll-up doors for entry/exit of large DUTs, such as industrial process equipment and vehicles. Integrating a full-scan height chamber into an existing building rarely is this straighforward. Often times, it requires significant compromises to optimum chamber dimensions.
Using in-house engineering and labor, the chamber was built in two stages. First, the shielded chamber was constructed and used as a shielded enclosure, without ferrite absorbers, due to a high demand from customers.
The second phase of the project consisted of selection, design, and installation of the ferrite-tile lining. Installation and testing were completed within two months, with most of the work done nights and weekends so that normal testing operations were not interrupted.
Shielding
A welded-construction method was chosen to ensure isolation levels up to 40 GHz. An unusual feature of the shielding was the use of large steel sheets pre-formed to eliminate all corner weld seams.
An extra measure of shielding integrity was added by backing the butted sheet joints with steel straps. This provided two additional weld seams for a triple-welded seam. All welds were checked for integrity by applying kerosene or dye penetrant to the seams prior to radiated isolation measurements. This was an easy way to locate any pinholes in the weld seams prior to radiated testing.
Measured plane-wave shielding isolation was >120 dB from 1 MHz to 18 GHz. Plastic underlayment with dielectric mounts for the tube steel framing was used to isolate the shielded enclosure from ground. Final isolation levels were measured at 20 MW @ 500 VDC.
Components
The Euroshield doors were among the few items not fabricated directly by lab personnel. Doors for manually operated personnel (2-m × 1-m) and for sliding equipment (2.7-m × 2.5-m) were fitted to opposite end walls. Halogen light fixtures were mounted in the upper four corners.
A bolt-in 0.065-m × 0.85-m bulkhead panel for cabling was recessed into the end wall adjacent to the small door. All electrical conduits and filters were located externally or beneath the raised ground plane floor. A 2-m dia, flush-mounted, air-powered turntable was located in the floor.
Absorber Considerations
Fully machined ferrite tiles were selected for their dimensional accuracy, appearance, and most importantly, their absorption performance. Representing more than 50% of the total project cost, Hermon consulted with several absorber vendors to learn the aspects of the installation and the performance capabilities of ferrite tiles.
The tile installation presented several problems not common in conventional modular wood-core shielded rooms. Modular walls typically are very smooth and flat, lending themselves to easy screw attachment of individual tiles or tile panels. For welded enclosures, the chamber industry normally maintains wall flatness to within 1/8″ of runout every 10’; however, even if the walls are perfectly flat, the weld seams protrude and require extra mounting preparation as shown in Figure 1.
To provide a flat planar surface, Hermon designers used a layer of RTV silicon between the steel wall and the wooden dielectric spacer. The dielectric spacer chosen was 13-mm-thick plywood attached to the steel wall by welded, threaded studs. The tiles then were bonded to the dielectric spacer with epoxy adhesive.
Special plastic bushings were designed to provide a positive mechanical mounting method for each tile. Metal screws were used with the bushings to secure each tile to the mounting structure without exerting undue strain on the fragile ferrite material. The screw attachment was an extra measure to keep the tiles securely attached. This level of installation preparation is much greater than typically required in anechoic chambers.
Consultation with the tile supplier resulted in a mounting method that was mechanically stable and improved the absorption performance of the ferrite tile. By incorporating a dielectric spacer between the tile and the metal surface as shown in Figure 1, the usable bandwidth of the 6.3-mm-thick tile was extended to above 1 GHz. The dielectric spacer can be any nonconductive material such as air, plywood, or particleboard. Figure 2 shows the effect of the spacer on return-loss performance of the ferrite tile mounted directly to a metal surface vs mounted with a 13-mm wooden spacer.
The chamber converts from a semi-anechoic to a fully anechoic type by using removable 60-cm × 60-cm panels on the ground plane. A plywood and vinyl covering on the floor panels protects the ferrite tiles from damage during use.
Test Results
The moment of truth came when Normalized Site Attenuation (NSA) measurements were performed in accordance with the volumetric procedure for alternate test sites contained in ANSI C63.4. Results in Figures 3 and 4 showed measured NSA performance over a 1.5-m dia × 2-m-high test volume along with the ANSI ±4-dB limit lines. No tuning posts or baffles were used.
While similar NSA results have been achieved using grid ferrite tiles, there are few, if any, conventional tile chambers that have achieved this performance without the use of hybrid foam absorber materials.
Conclusion
While the do-it-yourself approach may be economical, it does come with significant risks. Imagine building a chamber and then, after mounting $100k of ferrites, discovering that the NSA deviations are too large. Even reputable chamber designers must perform extensive computer modeling to accurately predict the field deviations in a chamber. Turnkey facilities cost more because all labor and risk are assumed by the chamber builder, so it is wise to consider all of the pros and cons before proceeding.
About the Authors
Tom Ellam is the product manager for Ferrite Tiles at Fair-Rite Products. He previously was the engineering manager with EMC Test Systems. Mr. Ellam also worked at Lockheed Missiles and Space after receiving a B.S.E.E. degree from California State University. Fair-Rite Products, One Commercial Row, Wallkill, NY 12589, (914) 895-2055.
Alex Usoskin is responsible for measurements and quality assurance at Hermon Laboratories. He graduated from Tel Aviv University with a degree in mathematics and computer science.
Gonen Usoskin is the marketing manager at Hermon Laboratories. He joined the family business in 1995 after receiving a B.A. degree in management at Tel Aviv University.
Edward Usoskin is the founder and executive manager of Hermon Labs. He received a Ph.D. degree in electronics from St. Petersburg University, Russia. Hermon Laboratories, Ha Takhana Rd., Binyamina 30550, Israel, 011 972 66 288 001.
Copyright 1997 Nelson Publishing Inc.
July 1997
