Radiated emissions testing involves measuring the electromagnetic field strength of the emissions that are unintentionally generated by your product. Emissions are inherent to the switching voltages and currents within any digital circuit, the only question is: how large are the emissions and do they comply with the emissions limits?
So how do EMC test labs measure radiated emissions?
Radiated Emissions Test Method
The electromagnetic waves don’t extend out from your product in a nice spherical pattern.
The emissions tend to be pretty directional, so a test lab has to vary the height of the receiving antenna between 1 and 4 meters as well as rotate a turntable.
The receiving antenna picks up both the signal direct from the EUT, as well as a bounce off the ground. To increase measurement accuracy, the ground is covered with an electromagnetically reflective surface (aluminum, steel, wire mesh etc..) and this ground plane must be relatively flat.
The test lab will scan the frequency band of interest and look for emissions that are close to the limits. Using a process called ‘maximization’ (described below), the test lab focuses in on each of these emissions, and quantifies the amplitude of the field strength.