What is the difference between conducted emissions and radiated emissions in an RF system?

Conducted emissions and radiated emissions are the two categories of unintentional electromagnetic energy that an electronic device generates. They are measured separately and have different compliance limits: (1) Conducted emissions (CE): unwanted RF signals that travel along the cables and wires connected to the device (power cords, data cables, control lines). The emissions propagate as AC currents and voltages on the conductors. Measurement: a Line Impedance Stabilization Network (LISN) is connected between the power source and the device under test. The LISN provides a defined impedance (50 ohms) and filters the external noise, presenting only the DUT emissions to the measurement receiver. Frequency range: typically 150 kHz to 30 MHz (FCC Part 15), or 9 kHz to 30 MHz (CISPR 32). Below 150 kHz: emissions are generally not regulated. Above 30 MHz: emissions transition to radiation and are covered by radiated emission limits. (2) Radiated emissions (RE): unwanted electromagnetic fields that radiate from the device, its cables, and its enclosure. The device acts as an unintentional antenna, with the cables often being the primary radiating elements (they are the longest conductors and the most efficient antennas). Measurement: performed at a specified distance (3 m, 10 m, or 30 m) using calibrated antennas in an anechoic chamber, OATS (open area test site), or semi-anechoic chamber. Frequency range: 30 MHz to 1 GHz (FCC Part 15 Class B). Extended to 6 GHz, 18 GHz, or 40 GHz for intentional radiators and 5G devices. Key relationship: conducted emissions on cables create radiated emissions. A 1 MHz conducted emission on a 1 m cable: the cable radiates as a monopole antenna. At frequencies where the cable is > lambda/10: the radiation is significant.