Electromagnetic Environment
Coupling
Interference reaches a victim by radiation through space, by conduction along wires, or by capacitive and inductive coupling between adjacent conductors; common-impedance coupling through a shared ground return is another path. Sources include motors, relays, switching regulators, digital clocks, transmitters and static discharge.
Mitigation
Shield and bond properly: a shield works only when bonded correctly, and a 360 degree backshell termination beats a short pigtail. Ground planes, twisted pairs, coaxial cable, screened looms, filters, ferrite beads and physical separation from antennas all reduce coupling. HIRF and lightning are severe external threats covered by qualification standards, and portable electronic devices can interfere with navigation and communication receivers.
Traps
Assuming a shield with a long ground lead works; forgetting that interference may travel on a wire rather than through the air; ignoring that fixing the coupling path can be cheaper and more effective than fixing the source; and assuming higher frequency is always worse, which depends entirely on the path.
Worked example
In the far field, doubling the distance from a radiating source reduces the field by about 6 dB, since 20 log 2 is approximately 6. A shield rated at 20 dB reduces the field inside to one tenth of that outside. Moving an antenna or rerouting a loom is genuine engineering, not guesswork.