avy planes that land on carriers are designed and built to withstand interference from a broad range of electromagnetic fields. In fact, the outer shell of the plane, as well as its internal electronic equipment and interconnect cabling are all designed to prevent penetration of disruptive electromagnetic signals - both those generated internally as well as those emanating from external sources.

Electromagnetic interference can jam sensitive equipment, burn out electric circuits, and even prompt explosions. Which is why drivers are directed to turn off car radios when travelling through mining areas where electronically controlled blasting is under way. In aircraft, EMI can affect everything from fly-by-wire flight control systems to a cockpit fuel gauge, and in extreme cases put a plane into an uncommanded dive or shut down a critical avionic system.

Whither EMI?

Historically, EMI has been a factor in aircraft construction since the 1930's when brass conduit was first used to shield cabling for newly introduced electronic communication systems from reciprocating engines and magneto ignitions. But such man-made electromagnetic "noise" generated incidentally by motors, generators, and other machinery turned out to be just one of the classes of EMI which would affect the safe operation of aircraft.

Naturally occurring radio noise originating from atmospheric disturbances (including lightning) and extraterrestrial sources (such as sunspots) can also degrade the performance of electronic equipment. Communications signals may also interfere with the operation of sensitive electronic equipment. To protect avionics systems from this class of interference, intentional radio frequency (RF) emitters like CB radios, remote-controlled toys, and walkie-talkies are banned outright on commercial airline flights. Most, but not all, airlines extend the ban to portable radios and television receivers.

Personal Electronic Devices (PED) such as laptop computers, hand held scanners, and game players, while not intentional emitters, can produce signals in the 1 MHz range and can therefore affect the performance of avionic equipment. As navigation cabling and other critical wiring runs along the fuselage inside the aircraft skin, it's no wonder that passengers sitting just a few feet away are warned about the indiscriminate use of such devices. Since the thin sheet of dielectric material that forms the inside of the passenger compartment - typically fiberglass - offers no shielding whatsoever; and since commercial passenger jets contain up to 150 miles of electrical wiring, it is extremely important for passengers to heed regulations on the use of disruptive electronic equipment.

Obviously, such internal sources of EMI are particularly dangerous to aircraft because they are so close to the systems they might affect. But external sources, such as radio and radar transmitters on the ground, or radar from a passing military plane, can be even more disruptive due to the high power and frequency of such equipment.

As if the many external and internal sources of EMI were not enough of a concern, a major EMI co-conspirator in aircraft is the aluminum airframe itself, which in certain circumstances can act as a resonant cavity, or a phased array. Behaving much like a satellite dish, the airframe can compound the effects of both internal and external EMI by concentrating transient signals and broadcasting the interference into nearby equipment.

Define Your Terms

By the early 1960's, interference problems had broadened to encompass the entire electromagnetic spectrum. And the phrase "EMI" was coined to describe electromagnetic interference in its broadest sense. Practically speaking, all emitters, receptors and frequency bands are thus part of the EMI definition. Consequently, such diverse problems as interference from ground loops, mismatched impedance paths, direct magnetic/electric field coupling (AC Hum), electrostatic discharge (ESD), power line conducted emissions and radiated emissions from other such sources all fall under the umbrella of EMI.

In scientific circles, the terms "electromagnetic compatibility" and "electromagnetic interference" are used almost interchangeably: EMC simply describes efforts to control or eliminate the problems created by EMI. For commercial electronic and electrical equipment the Federal Communications Commission (FCC) has regulations defining allowable emission and susceptibility levels. Military equipment is regulated by MIL-STD-461 and MIL-STD-462 (refs 4-10 and 4-11). MIL-STD-461 defines allowable emission levels, both conducted and radiated, and allowable susceptibilities, both conducted and radiated.

Radio Frequency Interference (RFI) is a special class of EMI in which radio frequency transmissions (usually narrow-band) cause unintentional problems in equipment operation. Radio frequency interference can originate from a wide range of sources, but precautionary measures generally focus on man-made sources including 2-way radios, pagers, mobile phones, and emergency and public safety communications systems. Power lines, transformers, medical equipment, electromechanical switches and many others unintentional emitters also produce RF energy. In common (unshielded) communications systems, RFI can degrade or completely disrupt signal quality, overall system performance, and system carrying capacity. In worst case scenarios, RFI can render an electronic system completely nonfunctional.

A recent incident on a commercial passenger plane illustrates the power of even low frequency RFI to disrupt avionic systems. In January 1993, on a flight from Denver, Colorado, to Newark, NJ, an aircraft lost all directional gyros (electromechanical devices that indicate orientation) at cruise altitude. The captain instructed the flight attendant to go through the cabin and tell all passengers to turn off their electronic devices. She reported back that about 25 passengers with portable radios had been listening to a Denver Broncos playoff game and that one passenger was also using a lap top computer. Within two minutes of the Captain's request to turn the radios off, the gyros had swung back to their correct heading. Later in the flight, several Bronco fans covertly resumed their use of the portable radios, and again the directional gyros began to malfunction. The radios were then confiscated and no further problems were experienced on the flight.

HIRF, or High Intensity Radiated Emissions (also known as High Intensity Radio Emissions) refers to emissions from radar, microwave, radio, and television transmitters, high power AM/FM radio broadcast systems, TV transmitters and other powerful communications systems. Recently, a great deal of public interest has centered on HIRF as a possible cause for the crash of TWA Flight 800, which apparently was in the proximity of a number of naval ships when it inexplicably crashed. Whether or not HIRF from the navy ships was the cause of the crash, the FAA issued a Flight Standards Bulletin about the problem of High Intensity Radiated Fields. The bulletin states that high powered electromagnetic interference can potentially lead to disruptions in airplane navigation and communication systems and to "loss of aircraft and life."

TEMPEST is an acronym for Transient Electromagnetic Pulse Emanation Standard. It is both a specification for computerized equipment as well as a term used to describe the process for preventing compromising emanations from electronic equipment. The fact that computers, printers, and electronic typewriters give off electromagnetic emanations has long been a concern of the US Government. A hacker using off-the-shelf equipment could potentially monitor and retrieve classified or sensitive information as it is being processed without the user being aware that a loss is occurring. To counter this vulnerability, the US Government has long required that electronic equipment used for classified processing be shielded to reduce or eliminate transient emanations. This is typically done by shielding the device (or sometimes a room or entire building) with copper or other conductive materials.

When EMI and Avionics Meet

The frequency bands used by avionic systems span the electromagnetic spectrum from a few kilohertz to several gigahertz. At the low end, Omega Navigation, which is used to fix aircraft position within a network of groundbased transmitters, operates in the frequency range of 10 to 14 KHz. VHF Omnidirectional Range Finders (VOR) are radio beacons used in point to point navigation. They operate from 108 to 118 MHz.

Glideslope Systems used during landings operate in the 328 to 335 MHz range. Distance-Measuring Equipment (DME), which gauges the space between the aircraft and ground-based transponders operate at just over 1 GHz. Also in the spectrum above 1 GHz are global positioning, collision avoidance, and cockpit weather radar systems.

Personal Electronic Devices (PED) operate at frequencies from 10 to 15 KHz for AM radios and up to 400 MHz for laptop computers. When the higher harmonics of these signals are taken into account, the emitted frequencies cover almost the entire range of navigation and communication frequencies used on the aircraft - and PEDs are just a single class of EMI emitters. When the full spectrum of other radiated and conducted EMI emitters are taken into account, it becomes clear that the entire system of electronic equipment aboard commercial and military aircraft are at risk to EMI.

But the fact that all avionic equipment and cabling which is critical to the functioning of commercial and military aircraft is shielded against EMI, raises an interesting question: how exactly does EMI, such as RFI from a passenger radio or walkman permeate the system?

In many cases the cause is simply inadequate shielding, or shielding which has been damaged during servicing or has degraded due to corrosion, thus increasing the resistance of the electrical connection to ground. As effective shielding is dependent on good grounding, any additional resistance in the system - for example at a corroded backshell or a poorly installed shield termination crimp ring - can enable the wires to pick up interfering signals directly.

Aircraft with navigation and communication antennas located outside the skin of the aircraft can also pick up EMI radiated through passenger windows and other unshielded openings in the plane. The pathway for RFI from a passenger PED would, in this example, be out the window, back into the plane via an unprotected or RFI sensitive antenna, and then directly into a navigation receiver, autopilot computer or other avionic device.

EMI Management

Effective shielding of avionic devices must anticipate both "radiated susceptibility" (the degree to which outside interference affects the reliable functioning of equipment) and "radiated emissions" (the extent to which the device itself creates electromagnetic waves which can affect its function). In both cases, the techniques for managing the interference include reflecting the signals outright, reducing entry points in equipment and cable shields, absorbing the interference in permeable material and dissipating it as heat, or conducting the EMI along the skin of the device/cable and taking it to ground.

In practical terms, EMI management is accomplished by plating the skins of cases and cable shields, building up the density (thickness) of shield material, or eliminating line-of-sight entry points through which electromagnetic waves can penetrate or escape.

The frequency of the interfering signal is a critical concern when designing an effective shielding solution. Low frequency magnetic waves in the 1 to 30 Khz range, for example, are most effectively shielded by absorbing the signals in permeable material. High frequency signals (30 KHz and above) are most effectively shielded by reducing entry windows and by insuring adequate surface conductivity to ground. In interconnect applications, wires and cables are typically shielded by placing a conductive material between the cable conductor and its outer jacket, or by covering individual conductors within a cable with shielding material. Again, the purpose of such shielding is to either capture the EMI and take it to ground or to dissipate the interfering signal as heat.

Shields must also be effectively terminated to the connector backshell lest radiation enter the system at the backshell/connector/shield interface and defeat the purpose of the shield.

Cable Shield Termination Accessories and technologies are available in a wide range of designs and constructions (see sidebar, pages 4 and 5). The relative effectiveness of each style, and for that matter of the complete shielding solution, can be measured using a transfer impedance test. The transfer impedance test is the most widely accepted absolute measure of a shield's performance. It is used to evaluate cable shield performance against electrostatic discharge and radiated emissions coupling at frequency ranges up to 1 GHz. This testing method is recommended by the International Electrotechnical Commission as well as the military.

This transfer impedence test illustrates how high frequency shielding is improved by adding multiple layers of tinned coppper shielding to Glenair made metal core conduit.

Shielding Solutions

Cable shielding is manufactured in a wide range of designs and configurations. Each type of shielding has advantages which must be considered when selecting the best and most cost-effective option for a given application. Common cable shield materials include:

Braided Shields: Braided shields provide exceptional structural integrity while maintaining good flexibility and flex life. Braided shields are effective at minimizing low frequency interference at audio and RF ranges. In use, the reduction of EMI is dependent upon the signal amplitude and frequency in relation to the many combinations of mesh count, wire diameter and the braid material. Generally, the higher the percentage of braid coverage, the more effective the shield against high-frequency emissions. Materials include tin-plated copper, nickel-plated copper and tin-plated iron/copper as well as hybrid materials such as metalized Kevlar (Aracon®).

Foil Shields: Foil shields are made from aluminum foil typically laminated to a polyester or polypropylene film. Foil shields provide 100 percent cable or component coverage, improving protection against radiated emission and ingress at audio and radio frequencies. Because of their small size, foil shields are commonly used to shield individual pairs in multiconductor cable to reduce crosstalk. Foil shields may also be bonded to a coaxial cable insulation or cable jacket with a layer of adhesive, allowing for faster, easier and more reliable termination. Used in combination with wire braid, foil can provide maximum shield efficiency across the frequency spectrum.

Metal-Core Conduit: The "Cadillac" of EMI shielding, helically-wound metal conduit provides extremely high levels of EMI protection across all radiation fields and frequencies. Metal-Core Conduit is the material of choice for TEMPEST secure communications and other applications involving sensitive electronic equipment and intense levels of EMI. Available materials include brass, nickel/iron and stainless steel. The conduit is generally specified with overbraids of plated copper and rubber jacketing. The choice to use Metal-Core Conduit depends on the sensitivity of the equipment under consideration and other mechanical, thermal and environmental requirements.

What About Fiber Optics?

Among their many other virtues, Fiber Optics are completely immune to EMI. Which means the evolution of fly-by-wire systems to fly-by-light systems not only expands data carrying capacity and reduces the weight of interconnect cabling, but it also eliminates the many risks and problems of electromagnetic interference. It is for this reason, among others, that so many retrofit projects are currently underway to replace traditional copper conductors with fiber. At Glenair, we're well positioned to support our customers with new fiber optic solutions, but also to continue to meet their needs for conventional EMI shielding technologies.