Radiated Emission
Radiated emission is noise spreading through the air as an electromagnetic wave. The source is often not the circuit itself but the long cables attached to it: an unshielded motor cable becomes an effective antenna for the high frequency noise it carries and raises the receiver's noise floor.
The radiating efficiency of a conductor depends on its length relative to the wavelength of the noise it carries. A motor cable a few metres long is a serious radiator for components in the tens of MHz. That is why most radiated emission problems are solved in the cabling rather than in the board design.
- Shielded cable: keeps the noise current inside the screen; terminating the screen matters as much as the cable itself.
- Cable length: unnecessary length and coiled-up excess increase radiation.
- Separation: routing power and signal cables on separate paths and crossing them at right angles reduces coupling.
- Antenna position: take the receiver antenna as far as possible from motor and driver cables.
The symptom is clear in the field: with the machine stopped the range is at the expected level, and it shortens once the motor starts running. This is not an output power problem but a noise floor problem, and the solution is on the cabling side.
Context where this term is used: Wireless remote control installation: pairing, relay, safety