5.14 Electromagnetic Environment Flashcards
What are the 3 elements of any electromagnetic compatibility issue?
Source, Receptor, Path Between them.
How are electromagnetic compatibility problems solved?
By identifying at least 2 of these elements and eliminating one of them.
What are the 4 methods of coupling electromagnetic energy from a source to a receptor?
- Conducted (electric current)
- Inductively (Magnetic field)
- Capacitively (Electric field)
- Radiated (Electromagnetic field)
At distances greater than one-sixth of the wavelength of the frequencies of concern what happens?
E (electric) fields and M (magnetic) fields COMBINE into EM Electromagnetic fields.
What is the most important protection from lightning strikes?
Electrical bonding
Describe “Noise”
Man man electromagnetic generated by motors, generators and other machinery.
Describe direct and indirect effects of lightning damage.
Direct = Structural damage
Indirect = Induced transients distrupting electrical equipment.
On an antenna what provides the lightning protection?
Mounting bolts create a path to ground.
What must we be careful with in regards to High intensity radiated field HIRF
They are man made sources of electromagnetic radiation generated EXTERNAL to the aircraft considered possible interfering with safe flight.
Define Electromagnetic Compatibility EMC
Capability of equipment to operate in intended area.
Define Electromagnetic environment.
Electromagnetic phenomena existing at a given location.
EMI (Electromagnetic interference) is defined by NATO as?
Any electromagnetic disturbance that interrupts, obstructs or otherwise degrades or limits the effective performance of electrons/electrical equipment.
What causes EMI Permeation?
Inadequate shielding or damaged shielding/corrosion
What is the most difficult EMI to detect?
Radiated electromagnetic energy entering an adjacent device.
What mechanisms are there for energy transfer to affected devices?
Radiation, Conduction and Induction.
When dealing with Radiated EMI what are the 3 options?
Remove “source”, Harden Target “receptor”, separate devices “path”
Effective shielding of avionic devices must anticipate
Radiated Susceptibility and Radiated Emissions.
What are some techniques for managing interference?
- Reflecting signals
- Reducing entry points.
- Absorbing in permeable materials.
4.Dissipating as heat. - Conducting EMI along the skin and shunting to ground.
How is EMI shielding achieved?
1.Plating the skins of cases and cable shields.
2. Building up the density of shield material
3.Eliminating line of site entry points.
What is the purpose of shielding?
To capture the EMI and take it to ground or dissipate signal as heat.
How are magnetic waves shielded below <30 KHz?
Highly permeable material.
How are magnetic waves shielded above >30KHz?
Reducing entry windows and ensuring good earths.
What are the benefits of Braided shields?
- Increased structural integrity
- Good flexibility/life
- Higher percentage of coverage = more effectiveness against high frequency’s.
Describe Foil Shields
- Made from aluminum foil laminated to polyester.
- 100% cable or comp coverage.
- Commonly used to shield individual pairs in cables to reduce cross talk.
Foil and braided shielding when used together can provide?
Maximum field efficiency across frequency spectrum
Multi shielding provides?
Superior attenuation that is effective from kilohertz to the gigahertz.
To prevent electromagnetic interference being radiated from signals present IN wires what should be the layout of braiding?
Inner = low perm material
Outer = high perm material.
The low permeability shield should always be closest to?
Source of interference.
Low permeability shield provides?
Good shielding at high frequencies.
High permeability shield provides?
Excellent shielding at low frequencies.
What methods are used to propagate EMI caused by Radiation?
Shielding, Filters & Ground.
What methods are used to propagate EMI caused by Conduction?
Wire routing & loops.
What methods are used to propagate EMI caused by Induction?
Distance and shielding.
What helps reduce EMI in a printed circuit board?
Continuous ground plane.
What is a completely enclosed conductive surface called?
Faraday cage.
What is the goal of the structure shielding?
Seal all cracks and holes to make it as close to a faraday cage as possible.
What effect does EMI have on fibre optics?
Nothing it is immune.
