Electronic Military & Defense magazine was developed for engineers, program managers, project managers, and those involved in the design and development of electronic and electro-optic systems for military, defense, and aerospace applications.
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Technology How To Avoid Electromagnetic Interference Hazards The damaging effects of EMI By Alan Lowne B ased on a true story, the movie Lone Survivor tells the story of four Navy SEALs on an ill-fated covert mission to neutralize a high-level Afghan Taliban operative in 2005. Ambushed by enemy forces, one of the SEALs climbs to higher ground to make an emergency call via a satellite phone — not a wise move since transmissions like this are easily detected and can pinpoint a soldier's position. Modern warfare acknowledges the vulnerability of continuous transmissions, and the technique of choice is to use bursts of short, packetized up/down data transmissions via satellite, doing subsequent information retrieval, evaluation, and creation in an electromagnetic interference (EMI)- shielded, undetectable environment. Complex electronic systems often create unwelcome EMI/radio frequency interference (RFI), which can interfere with the operation of critical communication, computer, command, and control systems. Almost all electrical and electronic equipment can create interference or emit an electronic signature that is detectable by those of unfriendly intent. Electromagnetic energy can escape from one product and interfere 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. 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 main supply voltage by adding harmonics at multiples of the 50 Hz or 60 Hz supply. These emissions can catastrophically interfere with emergency radio communication and aircraft navigation systems. This interference may not just be a radiated phenomenon, because it can also be distributed via a building's power wiring. Powerful nearby radar installations or MRI machines in field hospitals can create unwanted interference, too, disturbing operational activity of command centers. Security personnel are often required to ensure "EMI-clean" buildings or rooms, providing instant electromagnetic wave shielding to protect c o n f i d e n t i a l o r proprietary information on electronic equipment from interception by unauthorized persons outside the building. The information can be intercepted through the detection and analysis of electromagnetic waves emanating from the equipment or even from the audio pattern of voices intercepted by lasers trained on exterior windows. To protect against EMI-eavesdropping, government embassies, traveling diplomatic staff, industrial computer facilities involved in classified government contracts, and sensitive military communications facilities all need some level of electromagnetic shielding over a specified frequency spectrum. In all of the aforementioned scenarios, an instant EMI tent (Figure 1) is a reliable solution. Portable EMI shelters, which can be the size of a warehouse or an aircraft hangar, provide EMI and RFI shielding in excess of 90 dB using Ni/Cu/Ag woven material pulled over quick-setup support structures — the fastest to erect made with air- inflated supports. Configured to rapidly deploy and provide a fast, uncomplicated, state-of-the-art shielded enclosure, these environments not only shield against electromagnetic emissions, but they Electronic Military & Defense Annual Resource, 4th Edition 20 Figure 1: 15' by 20' EMI/RFI airframe enclosure with external foyer