Portable, custom-designed shielded enclosures made with attenuating fabric can create EMI-quiet environments almost anywhere.

 15’ x 20’ EMI/RFI airframe enclosure with external foyer. Photo Credit: SwordThe Greek philosopher Euripedes once said: “The good and the wise lead quiet lives,” and that is especially true in the world of electromagnetic radiation. Portable, custom-designed shielded enclosures made with attenuating fabric can create EMI-quiet environments almost anywhere, for situations ranging from pre-compliance testing, electromagnetic susceptibility, and secure communications. Designers of radio devices need to perform special tests, which must be done in a controlled environment, for RF immunity of parts to be used in a system. On a smaller scale, small RF isolation boxes and pouches make bench-testing procedures convenient and cost-effective, but shielded environments can be made big enough to hold a tractor or an entire aircraft.

RF signal reduction structures are also needed for command and control operations, as well as other applications where strength, weight, collapsibility, and portability are vital for successful and secure military field operations. High attenuation, RF signal-secure portable tent enclosures can be designed for safe communications where maximum signal attenuation is required with the flexibility of a free-standing, portable system.

Versatile, mobile RF enclosures provide superior RF shielding in a fabric-based structure, which reduces the strength of RF signal emanations from computers and communications equipment to a low level. This eliminates any possibility of electronic eavesdropping – an essential requirement for domestic and international intelligence activities.

In the commercial arena, nefarious economic tactics involve the unwanted retrieval of information – theft activities that are easy, safe, and lucrative unless they are guarded against. With eavesdropping laws hard to enforce, the advancements in electronics and optoelectronics have made communications interception relatively easy.

All electronic products are inadvertently radiating some level of signals whose base frequencies are in the range of microwave radiation, which penetrates many kinds of materials. With faster voltage transitions in digital hardware, more high-frequencies are emitted. A spy may only need to find one frequency to detect the digital signal information.

Controlled Environment

Another reason to use a portable Faraday cage is to remove unacceptable RF interference, which can come from products like electric motors, drills, poorly suppressed vehicle engines, high-power flashing lights, or nearby broadcast transmitters. Switching power supplies contaminates the main supply through conduction and emission, adding harmonics at multiples of the 50 or 60 Hz supply.

10’ x 10’ EMI/RFI enclosure with exterior support frame. Photo Credit: Select FabricatorsEMI enclosures are valuable for creating a controlled environment to insure that products are not susceptible to ambient electromagnetic interference, ESD, magnetic fields, mains power glitches, dropouts, and brownouts. A critical military application for these EMI-clean environments is the examination of electromagnetic interference levels that cause munition ordnance fuse/detonators to activate.

Inside buildings there is a need to provide electromagnetic wave shielding to protect sensitive electronic equipment from high level RF or radar signals outside the building, and to protect confidential or proprietary information from interception by unauthorized persons outside the building through the detection and analysis of the electromagnetic waves emanating from the equipment. A portable enclosure is less expensive than treating the building structure with EMI-reduction techniques.

Durable shelters should not only be chosen for their quick setup and strike-down, but also for their attenuation performance, preferred options (i.e. energy-efficient LED lighting), and lightweight aluminum support structures, which maximize mission payload. RF portable shielding solutions should also consider shielding effectiveness during physical entry and exit of an enclosure.

Instant Protection

Vestibule designs are employed for walk-in enclosures to preserve attenuation qualities during entry and exit activities. Almost-instant and uncomplicated state-of-the-art environments are now available with inflatable airframe support structures. Tested to meet the highest standards of quality and durability, these instant EMI/RFI protection environments prevent the interception and analysis of electromagnetic radiation to unauthorized persons seeking valuable information derived from compromising emanations from electronic equipment, automated information systems, and telecommunications systems. Air-inflated support beams not only provide fast set-up and strike time, they also minimize the manpower needed for set-up, are lightweight, and easy to transport. No extra parts or tools are required and the enclosures are simple to operate with minimal personnel.

Shielding effectiveness of Ni/Cu/Ag material tested to MIL-STD-285 “Attenuation measurements for enclosures, electromagnetic shielding, and electronic test purposes.” These inflatables can be as small as 7' x 7' or as big as an aircraft hangar, and are tested to meet the highest military standards of quality and endurance with an outdoor lifecycle in excess of 10 years.

Many of these structures are currently being used in the harsh desert environments of Iraq and Afghanistan. They are treated to be fire-retardant, and will remain inflated even after damage from small arms fire.

Considerations in specifying an EMI-shielding enclosure.Air conditioning or a fan and vent system can be EMI-shielded to preserve the EMI-quietness of the structure, and to help make the environment usable for extended periods in many locations. Other features include a double-seal door system with conductive magnetic or hook-and-loop tape closures, a shielded floor, a built-in through-connector panel with filtered power, and a network of connections to suit individual needs. LED lighting that does not compromise EMI measurements can also be specified.

Tent enclosures are usually constructed from multiple layers of conductive fabric, such as electroless plated silver etched directly on to ripstop nylon fabric fiber. Copper and nickel layers are then electrolytically plated over the silver along with an outer protective coating for durability, giving more than 90 dB attenuation of signals from 10 MHz to 18 GHz under MIL-STD 285.

Designed with a variety of environments in mind, these shelters can withstand 2 in/h of free-falling and blowing rain for 30 minutes without intrusion of water into the shelter; 10 lbs/ft2 of snow load for 12 hours without any damage; steady winds of 55 mph with gusts up to 65 mph for 30 minutes; temperatures of -40° to 135°F; more than 50 erect/strike cycles without structural damage; and blackouts. Interior shelter lights are also not visible during ingress/egress within 100 meters with the naked eye, or within 300 meters with night vision goggles.

Custom-required connectors with precision machined I/O panels and options to suit actual test procedures should be specified to maintain the shielding integrity of the enclosure at the locations of cable penetrations, electronic filters, or shielded cables.

Whenever there is a need to eliminate external RF radiation, or to protect or calibrate equipment on-site, portable EMI enclosures enable engineers to make quick adjustments on-site – and air-inflated enclosures are the fastest way to create quiet environments.

This article originally appeared in the January/February print issue. Click here to read the full issue.