Living a Quiet Life

Parent Category: 2014 HFE

By Alan Lowne 

Finding an EMI-free environment for testing purposes is often a challenge for designers and test engineers alike. But economical, custom-made shielded tents and enclosures can quickly create EMI-quiet environments for situations ranging from pre-compliance testing, electromagnetic susceptibility, and secure communications. Portable RF-shielded enclosures  are ideal for applications including wireless device development and testing, secure communications, satellite communications testing and isolation, EMC pre-compliance, medical and aerospace equipment shielding, cellular and computer forensics, as well as RFID shielding for commercial and industrial settings. 

Designers of radio devices often need to perform special tests for RF immunity of parts to be used in a system, tests which must be done in a controlled RF environment or shielded enclosure to prohibit distortion. On a smaller scale, small RF isolation boxes and pouches make bench-testing procedures convenient and cost-effective, while, on the other end of the scale, shielded environments can be made big enough hold a tractor or even an aircraft. High attenuation, RF signal-secure portable tent enclosures can be purpose-designed for communications applications where maximum signal attenuation is required with the flexibility that a free-standing, portable system provides. Portable RF shielded enclosures provide superior RF shielding in a fast set-up, fabric-based structure which eliminates RF signal emanations from nearby radiating devices. With clock times now below nanoseconds, computers are radiating signals whose base frequencies are in the range of microwave radiation which penetrates many kinds of materials. With faster changes between two voltage levels in digital hardware, more high-frequencies are emitted.

Faraday Cages

Almost all electrical and electronic equipment, as well as nearby broadcast transmitters or motorized equipment, can create interference or emit an electronic signature. Solving both issues is addressed by the availability of quick set-up portable Faraday Cages. Radio interference can be caused by sources such as electric motors, drills, poorly suppressed vehicle engines, high power flashing lights, or nearby broadcast transmitters. Switching power supply circuits, used with many electronic products including lighting systems, are famous for contaminating the mains supply voltage by adding harmonics at multiples of the 50 Hz or 60 Hz supply. While merely irritating for consumers, these emissions they can catastrophically interfere with emergency radio communication and aircraft navigation systems, for instance. This interference may not just be a radiated phenomenon, but it can also be distributed via the building’s power wiring.

A prime application for EMI enclosures is to ensure that products are able to function correctly in their designed environments, and are not susceptible to, and immune from, ambient electromagnetic noise and interference. And not only for radiated and conducted RF requirements, also to be considered are ESD, magnetic fields, mains power glitches, dropouts and brownouts, all of which must be tested in a controlled environment. One military application for these EMI-clean environments is to examine what level of electromagnetic interference causes munition ordnance fuse/detonators to activate.

Electromagnetic interference (EMI) is electromagnetic energy that escapes one product and interferes with another. Signals can be radiated via an unwanted antenna extending from (or appearing in) the product—connecting leads or gaps in cases, for instance. Energy conducted via the product’s power lines can seep into the power terminals of another product. Conducted EMI is usually measured from 150 kHz to 30 MHz and radiated EMI is measured from 30 MHz to 1 GHz or to 5x the highest clock frequency, whichever is higher. Electromagnetic compatibility (EMC) refers to design philosophies that ensure a product is not interfered with by other products as a result of electromagnetic radiation or conduction; external electromagnetic radiation coming from another source does not cause a product to malfunction or affect it adversely.

Fabric Shielding

RF portable fabric shielding solutions also need to consider shielding effectiveness during physical entry and exit of an enclosure. Vestibule designs are often employed for walk-in enclosures, since opening the door can degrade immediate shielding effectiveness, resulting in possible data capture compromise. With the addition of a vestibule that is integral to the enclosure, the shielding environment is kept uncompromised for consistent shielding performance. The vestibule is usually designed with its own separate conductive floor, door and inner walls apart from the tent itself which allows maximum isolation for continuous testing or examination. Including a white ESD liner and ventilation in the design is beneficial since several people may occupy a RF EMI shielded tent at the same time. Air conditioning or a fan and vent system can be EMI-shielded to preserve the EMI-quietness of the structure, but make the environment usable for extended periods in many locations. A white ESD liner not only protects against static; it can also help technicians read and record data against a neutral, bright background. A double-seal door system, a shielded floor, and a built-in through-connector panel with filtered power and network connections to suit individual needs are also common requirements. 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.

Even inside buildings (or hotel rooms) there is a need to provide electromagnetic wave shielding, either to protect sensitive electronic equipment operating inside the building from high level RF or radar signals outside the building, or to protect confidential or proprietary information being processed on electronic equipment inside the building 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 often much less expensive than treating the building structure with EMI-reduction techniques.

Examples of the former condition include computer facilities located near military installations, and Magnetic Resonance Imaging (MRI) facilities located near a commercial radio broadcast station, and of the latter situation: government embassies and traveling diplomatic staff, or industrial computer facilities involved in classified government contracts. In both cases some level of electromagnetic shielding is required over a specified frequency spectrum. The owner, or user, of the building determines this shielding requirement based on an analysis of the potential problem. This analysis should include a site or computer equipment survey. When associated with a government installation, certain regulations and guidelines must also be followed to determine the shielding requirements.

Portable Shelters

Durable shelters should be chosen not only chosen for their quick setup, but also for their attenuation performance, together with preferred options such as energy-efficient LED lighting, and lightweight aluminum support structure which maximizes mission payload. Many of the portable shelters are rapidly deployable by land, sea or air without any additional pallets and tie downs. Instant and uncomplicated state-of-the-art environments are now available with inflatable air-inflated support structures. Tested to meet the highest standards of quality and durability, these instant EMI/RFI protection environments quickly prevent intercept and analysis of electromagnetic radiation to unauthorized persons seeking valuable information derived from compromising emanations from electronic equipment, automated information systems, and telecommunications systems. Usable almost anywhere, these quick-inflate enclosures provide the vital security needed for sensitive electronic equipment. Air-inflated support beams give not only the fastest set-up and strike time of any shelter; they minimize the manpower needed for set-up, and are lightweight for easy transport. No extra parts or tools are required and the enclosures are simple to operate with minimal personnel.  

Designed with a variety on environments in mind, shelters can be designed to withstand typical specifications such as: 2”/hr of free falling and blowing rain for 30 minutes without intrusion of water into the shelter; 10 lbs/sq.ft of snow load for 12 hours without damage; steady winds of 55 mph with gusts up to 65 mph for 30 minutes; temperatures of -40degF to 135degF; more than 50 erect/strike cycles without structural damage; blackouts—interior shelter lights 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. Just as the enclosure shielding design is the last line of defense for radiated EMI control, I/O filtering is the last line of defense for controlling conducted EMI. Higher filter insertion loss levels are often required for military equipment operating in severe electromagnetic environments or mission critical scenarios.  

Considerations in specifying an EMI-proof enclosure include:  operational environment, mobility, temperature/humidity, level of attenuation, erection and strike time, transportability, snow load, wind load, moisture permeability, illumination, air-conditioning, type and number of I/O connections, EMI standards to be adhered to, etc. Some materials are available that can withstand explosions, and can even be self-healing and sealing to some extent.

Whenever there is a need to calibrate equipment on-site, portable EMI enclosures enable engineers to make adjustments on-site, on-time. Staff can perform measurements on very sensitive equipment without any external interference. As the Greek philosopher Euripides said: “The good and the wise lead quiet lives” – so here’s to a “quiet life”!

About the Author:

Alan Lowne is CEO  of Saelig Company Inc. (saelig.com).