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A PCB trace can act as an unintenTIonal antenna even at lengths shorter than λ/4， increasing both emissions and suscepTIbility. Surface traces also exhibit this wavelength shortening effect， as one side of the dielectric serves to change the overall permitTIvity of the transmission.
Unintended antennas， such as PCB traces， are the key culprit behind radiated noise in digital systems. As you will see， the Class D audio amplifier is， in essence， a digital system from the perspecTIve of radiated emissions. One key principle underlying electromagnetism is that of reciprocity. A flow of current can create an electric field， and a change in electric flux can induce a current to flow. Likewise， an antenna that is good for receiving is also good for transmitting. If any of the dimensions of an unintentional antenna approach quarter wavelength at the frequency at which that antenna is excited by noise current， radiated emissions can be expected.
