Conducted Emissions vs Radiated Emissions

Can anyone justify why the frequency band for Conducted Emissions is set lower than that for Radiated Emissions in EMC regulations? Thanks!

Hello,

You may know that radiation from a wire carrying a current depends heavily on the size/wavelength ratio. A 3 cm wire segment (practically, think of a small loop) doesn’t radiate much when driven with 10 MHz. The same segment driven with source with 3 GHz will radiate more.

As at low frequency (say up to 30 MHz) many equipment (DUT) has a case with size << wavelength, most radiation is from the wires that go to the equipment. As there is some relation between the common mode current and the actual radiated power, it is easier to measure the common mode current (or voltage) instead of field components.

Measuring real radiated power at low frequency requires a large measuring site or independent E-, and H-field measurement including phase and orientation. Also common mode current may travel large distance through cables before being radiated, so in fact the DUT is very large and spot measurements of E-, H-fields do not represent the radiated power (far field radiation).

At high frequency (short wavelength), most radiation emanates from/via the enclosure or from the first decimeters to meters from wires connected to the equipment. This is especially true for systems in plastic housing for frequencies above say 200 MHz. Radiation may emanate from the PCB directly (think of CPU clock signals and/or the harmonic components). At a distance > 0.5 lambda and not too large DUT (for example 10 m measuring distance for 30 MHz and upwards), one can measure the real radiated power using (directional) antennas.

In case of large DUT, measurement at 3m (that is done frequently) introduces errors. Measuring at 10 meter reduces the error in case of large DUT as the source (the DUT) behaves more or less like a point source.

Thanks alot!!!