Testing EMC requires access to an electrically isolated chamber called an anechoic chamber. From here a special array of antennae can be used to measure the devices radiated emissions, spectrum analyzers are used for conducted.
In the anechoic chamber, they will test radiated emissions (interference that travels through the air) and conducted emissions (interference that travels through cables) with an antenna.
For radiated emissions tests they will test the DUT at different heights and directions to find the point in the air where the in-air interference is the worst.
For radiated emission testing the DUT is rotated in the horizontal plane and at each 45 degree point an emissions measurement is taken. The receiving antennae is horizontally aligned, that is, optimal for reception of horizontally polarized EM waves.
The antennae is then vertically aligned and the DUT is then rotated again and readings taken at every 45 degree point.
The resulting graph for emissions level vs frequency derived for the report is comprised of the highest readings at each frequency for in each of the 16 orientations.
Emissions above the compliance level are typically detailed in accompanying notes, stating the frequency and the polarization of the peak(s).