B1-05.14 ELECTROMAGNETIC ENVIRONMENT

Electromagnetic Environment Terminology

• Electromagnetic Environment (EME) is the complete set of electromagnetic phenomena at a specific location.
• Electromagnetic Compatibility (EMC) ensures equipment operates efficiently in its intended electromagnetic environment without causing interference.
• Electromagnetic Interference (EMI), as defined by NATO, disrupts or degrades electronic/electrical equipment performance.
• High-Intensity Radiated Field (HIRF) refers to man-made electromagnetic radiation sources external to aircraft.
• Radio Frequency Interference (RFI) is EMI within the radio frequency spectrum, caused by external disturbances affecting electrical circuits through induction, coupling, or conduction.

Avionic Frequency Bands

• Avionic systems utilize frequencies from a few kilohertz to several gigahertz.
• VHF Omnidirectional Range (VOR) operates from 108 to 118 MHz, used for point-to-point navigation.
• Glideslope systems for landings operate in the 328 to 335 MHz range.
• Distance-Measuring Equipment (DME) operates just over 1 GHz, measuring distance between aircraft and ground transponders.
• Above 1 GHz includes global positioning, collision avoidance, and cockpit weather radar systems.

PED Frequency Bands

• Personal Electronic Devices (PEDs) operate from 10 to 15 KHz for AM radios to 400 MHz for laptop computers.
• Emitted frequencies from PEDs cover most navigation and communication frequencies used on aircraft, making electronic systems vulnerable to EMI.

EMI Permeation

• EMI can permeate systems due to inadequate or damaged shielding, often worsened by corrosion, increasing electrical resistance to ground.
• Effective shielding requires good grounding; any additional resistance can allow wires to pick up interfering signals.
• EMI can enter aircraft through passenger windows and unshielded openings, affecting navigation receivers and autopilot computers.

Electromagnetic Interference (EMI)

• Electromagnetic interference radiating from carrier equipment triggered a pilot's emergency ejector seat in a British Harrier Jump Jet in the 1980's.
• The North Atlantic Treaty Organisation (NATO) defines (EMI) as an electromagnetic disturbance which interrupts, obstructs or otherwise degrades the effective performance of electronic or electrical equipment.
• Aircraft are designed to withstand interference from various electromagnetic fields, with the outer shell, equipment, and cabling designed to prevent signal penetration.
• EMI can disrupt sensitive equipment and circuits; in aircraft, it affects fly-by-wire systems, fuel gauges, and critical avionics.

Electromagnetic Compatibility (EMC)

• EMC is the ability of equipment to function satisfactorily in its electromagnetic environment without causing disturbances to other devices.

Elements of an EMC Problem

• A source of electromagnetic phenomenon
• A receptor (or target) that cannot function properly due to the electromagnetic phenomenon
• A path between them that allows interference.

Potential Sources

• Radio transmitters, power lines, and anything that utilizes/creates electromagnetic energy.

Potential Receptors

• Radio receivers, electronic circuits, appliances, people, and objects that utilize/detect electromagnetic energy.

Coupling Path

• Conducted (electric current)
• Inductively coupled (magnetic field)
• Capacitively coupled (electric field)
• Radiated (electromagnetic field).

Electric and Magnetic Fields

• Electric fields can exist with one pole, unlike magnetic fields which require two poles.
• Electric fields travel in straight lines from their origin, while magnetic fields curve from north to south.
• Electric (E) fields are created by voltages on conductors, and magnetic (M) fields by flowing currents; both field types are created by electrical signals in conductors.

Leakage and Antenna Effect of Conductors

• Common frequencies range from AC power lines to mobile phones (up to 1.8 GHz) with real spectrums being quite busy.
• Mains rectifiers can emit switching noise at harmonics, depending on power.
• Switch-mode converters operate between 2 and 500 kHz and can have emissions up to 1000 times their switching frequency.
• At distances greater than one-sixth of the wavelength of concern, E and M fields become full electromagnetic (EM) fields. Conductors are antennas, converting electricity into electromagnetic fields and vice versa.

Natural Sources of EMI

• EMI was a factor in aircraft construction by the 1930s, with brass conduit shielding communication systems from reciprocating engines.
• Atmospheric disturbances (lightning) and extraterrestrial sources (sunspots) degrade the performance of electric equipment.

Lightning Strikes and Lightning Protection

• High currents from lightning can cause structural damage (direct effects) and induce transients that disrupt equipment (indirect effects).
• Lightning damage can cause expensive downtime/repairs.

Electrical Bonding

• Bonding helps conduct lightning currents safely through the airframe, achieved via normal joints.
• Separate bonding may be needed for external parts.
• Antennas and probes in lightning strike zones should safely transfer currents to the airframe to prevent surges through cables.

Man-Made Sources of EMI

• RF Emitters: Communication signals can interfere with sensitive electronic equipment.
• Intentional radio frequency (RF) emitters like CB radios and toys are banned on commercial flights to protect avionics.

Personal Electronic Devices

• PEDs can produce signals in the 1-MHz range, affecting avionics.
• Passengers are warned about PED use due to the proximity of cabling to passenger compartments containing up to 150-mi of electrical wiring.

External RF Issues

• External radio and radar transmitters can be disruptive due to high power and frequency.

Airframe Effects

• The aluminium airframe can act as a resonant cavity, compounding the effects of internal and external EMI by concentrating signals.

High-Intensity Radiated Field

• High-Intensity Radiated Emissions (HIRF), also known as High-Intensity Radio Emissions, emissions from radar, microwave, radio and television transmitters, airport and weather radar, and similar systems
• HIRF encompasses man-made electromagnetic radiation sources external to the aircraft and considered as possibly interfering with safe flight.
• HIRF does not include interference among on-board systems; this type of interference is referred to as an EMC issue, EMI effects caused by PEDs carried by passengers or effects of lightning or of static electricity generated on the aeroplane.
• Systems above 400 MHz are generally narrow in beam width and PULSED such as telemetry, command and control and telemetry signals.

EMI Management

• When electromagnetic interference is suspected, the first step towards resolving the problem is to determine the mechanism used for energy transfer to the affected device(s):: Radiation, Conduction, Induction.
• Three options for facility dealing with radiated EMI: Remove (or reduce) the source, 'Harden' the target or Separate the devices (remove the path). Effective shielding of avionic devices must anticipate:
• Radiated susceptibility – the degree to which outside interference affects the reliable functioning of equipment
• Radiated emissions – the extent to which the device itself creates electromagnetic waves which can affect its function.

EMI Shielding

• Protection and containment of RFI is achieved with a foil wrap followed by good coverage braided shield.
• Crosstalk is prevented by isolating/shielding or alternating lengths of wires.
• Electromagnetic interference, low frequency grounding can be prevented by ferrous earth metal braid along with quality grounding wiring.
• High Voltage Transients are best eliminated by employing a non-ferrous shielding along with proper isolation wiring of equipment.

Cable Shielding

• Conical Ring Style Backshells provide reliable shield termination securing shielded cables under pressure utilizing pressure from shield components.
• Braided shields minimize low-frequency EMI, dependent on signal amplitude and mesh count parameters, tin coated braids are extremely effective.

Foil Shields

• 100% coverage is achieved utilizing aluminium layering laminated with poly film , foil shields are generally utilized to reduce crosstalk with individual pair layering over the cable component.
• Foil,shielding and braid is quite effective across most frequency spectrums when wiring and layering cables

Multi-Shielding

• When the use of component and cable shield are implemented this is known as multi-shielding and provides superior attenuation and is quite effective with conductors that are sensitive

Multi-shielding implementation offers a wide field of performance and implementation

• Protects conductors from external sources
• Armor implementation and protection
• Minimize special and segregation routing

EMC/EMI Problems

• Radiation is best combatted by increasing the shield both internally and externally along, with radio frequency filters and considering the effectiveness of the ground (Ohms)
• Induction mitigation methods included replacing the cables with shielded types, rerouting cables, and increasing distances

EMI Reduction Options

• EMI can be eliminated or reduced utilizing suppression technologies and techniques. Transmit antennae along with bandwith limits and electronic filters can assist greatly in reduction of interference.