How to Sustain the Silence of the Shielded Rooms

Shielded rooms, those sometimes peculiar-looking structures, play a major role in helping electronic products meet global EMC regulations. More specifically, it is in the silence of these rooms that engineers perform the noise tests needed so today’s sophisticated equipment complies with the current emission and susceptibility standards.

Fortunately, the size of the product to test is not an obstacle. Shielded rooms can accommodate products as small as computer parts or as large as a jet aircraft. And the noise that can be detected in enclosures ranges from DC to about 10 GHz.

Both permanent and modular enclosures are available. The permanent model requires fewer seams than a modular unit but cannot be relocated. Modular rooms can be moved, but the assembly clamping system can have deleterious implications on its long-term reliability if it isn’t maintained properly. It also has more seams than the permanent enclosure.

Less is better for seams in a shielded room, especially for the floor, because it is the one area where it is difficult to avoid moisture, said Bill Curran, President of Lindgren RF Enclosures. Moisture degrades the performance of the clamping system. Fortunately, mechanically seamless floors with modular walls and ceilings are available.

Permanent systems employ three joining methods: welding, soldering and gluing of homogenous material, said Mr. Curran. The performance of a permanent joining system is determined by the shielding effectiveness (SE) of the doors and penetrations.

When the shield material is a homogenous metal without inherent deterioration characteristics, the seams and penetrations are the weak links. For composite materials, most suppliers build in a margin of safety to compensate for any inherent deterioration characteristics of the material.

As a rule of thumb, Lindgren uses a 20-dB margin of safety between the inherent electric field SE of the materials, seams and penetrations and the requirements of the total shielded enclosure system. The SE of the enclosure can be calculated with the equation:

n

SE¢ dB = -20 log10[log-110(-SEdB/20) + S log-110(-LidB/20)].

i=1

where: SE¢ dB = Shielding effectiveness due to combined effects of base metal shielding effectiveness and leakage.

SEdB = Shielding effectiveness of the base metal.

LidB = Leakage present in the shielded enclosure.

The equation indicates that as the SE of the base metal improves, so does overall shielding. Conversely, as the leakage increases, the shielding decreases.1 Most users understand this equation and its implications for loss of SE around damaged or poorly maintained door-sealing mechanisms. But many users do not realize that the equation also refers to the loss of performance from poor quality seams or inferior assembly techniques, said Mr. Curran.

Welded systems are long-lasting, durable and strong, pointed out Joe Orloff, Director of Engineering at Rantec. Welded systems, however, do have drawbacks. They take longer to erect and the structures cannot be redesigned or relocated once they are built.

Mechanical systems are easier to erect and more economical than welded systems, continued Mr. Orloff. However, moisture is difficult to control, and special joint treatments are required above 16 GHz. Soldering is no longer a common technique because it is labor-intensive, subject to cracking, and does not provide adequate performance.

Mechanical joining offered by some companies, such as Lindgren, include a double electrically isolated system with two seams per joint and particle or plywood construction with four seams per joint. Both systems offer RF seal redundancy to improve performance and long-term reliability, but the double-insulated type has less than half the number of seams and two independent clamping systems, and is less sensitive to moisture.

The quality of an RF seal can be influenced by the clamping pressure applied at the seal, the capability of the chosen RF shielding materials to provide high conductivity, and the effects of corrosion and oxidation, said Mr. Curran. The clamping pressure for a plywood cell-type system depends on the amount of pressure from one fastener applied to the inside and outside surfaces.

If moisture is introduced, the plywood composite core expands and the ridged clamping system will take a set, causing a loss of SE when the moisture evaporates and the core material returns to its original thickness. You can get around this by using a resilient clamping system that tolerates expansion and contraction without losing seam integrity.

The shielding material should be a highly conductive material, such as copper and its alloys. Copper is more expensive than galvanized steel and aluminum but it retains surface conductivity better under normal usage, added Mr. Curran.
Preventive Maintenance

Shielded enclosures lose some of their RF shielding integrity over time and can develop leaks that render them ineffective. Many leaks are associated with devices that penetrate the RF barrier, including power-line filter feed-through pipes, AC and drain pipes, communications lines and fiber-optic wave-guide penetrations.

The screws that hold the shielding panels together are another source of leaks. Screws loosened by as little as a one-quarter turn can cause RF energy to escape.

Finding leaks around the perimeter of the shielded room should start with the door. Specifically, begin with the fingerstock because leaks often are caused by dirty fingerstock in the mating surface of the door. Loose clamping hardware on the door frame is another culprit that should be checked.

Aside from the door, check the wave guides, setup panels and any other place where openings exist, said Mr. Orloff. Rantec recommends you follow the manufacturer’s maintenance requirements and allow only personnel familiar with shielding to perform repairs.

Do not add supports, hangers or other non-RF items to the room, exterior walls or any other part of the structure. They can introduce leaks to the structure.

A visual examination of a shielded enclosure is second only to an SE test, said Mr. Curran. In the absence of an SE test, a thorough visual examination yields much information. Begin by checking the door for damage or a buildup of dirt at the contact seal surfaces. The door is the active component of the enclosure and is more likely to need repair or maintenance, depending on the use and abuse it receives.

Check the contact sealing surfaces to determine if they need to be repaired or replaced. Next, look for any holes or penetrations that have been added to the enclosure since it was installed and tested.

Air balancing is another check, but it should not be a problem if the enclosure is properly designed and assembled, continued Mr. Curran. An overabundance of positive or negative air pressure can affect the force needed to open and/or close the door.

Sometimes this problem can be caused by the heating/cooling system. Too much positive pressure in a solid, as opposed to a screen, RF enclosure can be alleviated by simply adding a pressure-relief vent. The only filters on an enclosure that require maintenance are the wave-guide air vents. In most cases, these can be vacuumed to remove accumulated buildup of dust or dirt.

Most degradation on a shielded enclosure occurs where an environmental stimulus accelerates the degrading effect. For example, clamping hardware on a door where shock and vibration are present or clamping hardware near honeycomb ventilation panels that experience temperature and humidity extremes are elements that require rigid preventive-maintenance schedules.

Moisture condensation caused by forced-air cooling or the trapping of moisture beneath the enclosure floor is yet another problem area. Mechanical stress points requiring close observation are the clamping hardware that joins the floor and wall panels. To correct loose clamps, remove them and clean all the mating surfaces or replace the clamp.

Use a gentle agent to clean and remove the oxides from the surface. For example, an aged surface of a zinc-electroplated clamp appears dull and may have a fine white powder on the surface. Clean the surface until there is a slight sheen with no trace of powder.

Be careful not to use a cleaning solution that will remove the electroplating from the clamping hardware or the galvanizing from the shielded panels, because then corrosion can occur. Once cleaned, avoid contact with your bare hands, because skin oils can accelerate the corrosion process.
Shielding Trends

Today, more and more RF doors and modular enclosure systems are being used for shielding, said Mr. Curran. The varieties continue to grow, allowing you to mix and match components.

The newer modular systems will be easier to assemble, continued Mr. Curran. This is important for small modular enclosures that need skilled personnel to perform accurate assembly to get the required performance characteristics. You can expect to see modular RF shielded enclosures made from self-contained RF panels with no loose parts.

Modular rooms are more popular now because of the immunity requirements of IEC 1000-4-3 (formerly 801-3), said Mr. Orloff. Customers are using compact, modular chambers to perform on-site precertification testing before they send their products to a test house for final testing. These new modular test chambers save space, have an ultra broadband frequency range and provide excellent site-attenuation characteristics.

Reference

1. White, D., A Handbook on Electromagnetic Shielding Materials and Performance, Don White Consultants, Inc., Gainesville, VA, 1980.

Shielded Room Products

Lightweight and Portable Room
Made of Conductive Fabric

The P2000 EMC Chamber is a maintenance-free system with a modular steel floor that clips together and a solid door with EMI-resisting windows. It has the durability of a solid screened room and the portability of a tent. Large objects such as vehicles can be built or moved onto the ground plane prior to installing the hood. Standard features include N-type power filters and a BNC coaxial input. The facility can usually be erected in four hours. Tempest Security Systems, (813) 884-4994.

Screen Enclosures Use
Two Conductive Barriers

The Double Electrically Isolated Screen Enclosure utilizes two conductive electromagnetic barriers that are electrically isolated. They are offered with bronze or copper screening and are available in a modular or custom design. Standard units are constructed with prefabricated panels and include a 3’ x 7’ door, a ground stud, a filter entry panel and a vinyl tile floor. Modular enclosures are assembled and tested prior to delivery. Lindgren RF Enclosures, Inc., (708) 307-7200.
Absorber Has Usable Frequency
From 30 MHz to 40 GHz

FerroSorb is a low-profile, hybrid anechoic absorber that has a usable frequency from 30 MHz to 40 GHz with an average return loss of -20 dB. It can be used to meet IEC 1000-4-3 immunity and ANSI C63.4 emission measurement standards. The material has a height of 41 cm. The absorber uses a multilayer dielectric with a blunt-tipped shape. Rantec Microwave & Electronics, Inc., (512) 835-4684.
EMI/RFI

Copyright 1995 Nelson Publishing Inc.

September 1995

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