Shielded rooms go portable

Tests for electromagnetic compatibility can benefit greatly from a shielded enclosure that blocks external signals. Such an enclosure may not be practical to build, though, especially if you have only an occasional need for it. Now, you have an alternative: the portable shielded enclosure.

Made of a metallized fabric, this tent provides a portable alternative to traditional screen rooms for shielding equipment. Courtesy of Select Fabricators.

An increasing interest in compliance testing, generated by the current wave of wireless system development, has fueled the need for such enclosures. Because test engineers typically perform precompliance tests to verify their wireless designs before submitting products for final certification by an outside agency, the demand for shielded enclosure space has grown. Renting such space has become more difficult, yet constructing a traditional screen room to perform the tests may not be a viable option for a company.

"We had a recent request from a company that was operating out of a leased building," said Dan Ramich, an engineer at Select Fabricators. "They did not want to build a screen room, because they would not be able to get back that capital investment if they moved."

Compliance testing is not the only application for the enclosures. Ramich noted that his customers have used the enclosure for a variety of applications where shielding is essential, such as when sensitive medical diagnostic equipment must be used in electrically noisy environments or when the military wants to prevent electronic eavesdropping on field computer installations. Even law enforcement has benefited, according to Ramich, who described a case where the police had obtained a cell phone with vital evidence in its memory. They placed the phone in a small shielded enclosure to block any calls that might trigger a memory erasure.

The raw material that Select Fabricators uses for its enclosures is a metallized nylon fabric from Statex that has been clear-coated to add scratch, tear, and cracking resistance. The exact mix of metal varies according to the desired frequency range and attenuation, and it can include copper, nickel, and silver. A typical sheet offers an average of 86 dB of shielding from 20 MHz to 18 GHz. A finished tent typically offers shielding from 65 dB to 70 dB between 14 MHz and 1 GHz and averages 50 dB up to 10.5 GHz.

The construction of the enclosure from the fabric sheets is a bit of an art, according to Ramich. Simply sewing a tent together from conductive fabric will not produce adequate shielding. To be effective, the shielded enclosure must have no breaks or gaps in its conductive material. Joints between fabric pieces thus represent a potential "hole" through which RF energy may leak. The method for joining the fabric pieces while maintaining shielding integrity is so critical for ensuring the enclosure's effectiveness that Select Fabricators treats it as a trade secret.

Tent fabricators have developed many other tricks for turning cloth into an electromagnetic shield. To allow cables to penetrate the enclosure wall, for instance, they have created shielded sleeves that are permanently attached to the tent.

Ramich explained that the sleeves are bunched up "like an elephant's trunk" so they can remain compact, yet be extended as needed. The cable passes through the sleeve to a termination panel outside the tent, where the sleeve terminates to the panel and a filtered pass-through connector carries the cable's signals past the shield. Similar innovation has been applied to the design of doorways and ventilation. In addition, fabricators have developed multilayer versions of the tents to offer increased shielding effectiveness.

The fabricators even offer accessories. Ramich noted that his company has developed a carry bag for enclosures so customers can readily transport them for field use. The bag holds both the folded tent and the support frame pieces.