Near Field vs. Far Field Radiated Emissions

Figure 1. The near-field emissions we measure
in the lab may not be an accurate measure of
the far-field emissions on which EMC is certified. 

When we are testing a product on our bench top for EMC, we are in close vicinity to that product in a typically noisy environment. All we can measure is the near field, the electric or magnetic field strength in close proximity to our product. It’s important to keep in mind that near field measurements are not the same as the far field (3 m or more) measurements in the FCC Part 15 Radiated Emissions test described earlier, and here's why. 

When we have some source that's generating radiated emissions, depending on the current distribution and the shape of that source, we will have all sorts of multipole moments radiating: dipoles, quadrupoles, hexapoles, etc. There will be different shapes of electric field radiating based on the pattern of loops that the currents make. They all radiate, but the radiation field from the different multipoles drops off with distance at different rates. It's the dipole field that keeps on going and contributes to the far field radiation field. And so unfortunately, when we’re close to the product, we’re measuring all the near field strengths, which we’ll rarely see in the far field because they drop off so quickly.

That means that we have to be very careful interpreting the results from near field measurements. The near field is going to give us the radiation from all the moments, not just the dipole terms. The downside is that what is measured in the near field is not always a representation of what will be measured in the far field. A lot of near field radiation may not be indicative of FCC failure. On the other hand, if we fail an FCC test, then we absolutely will see near field emissions, so anything that can be done to reduce the near field emissions will help with passing an FCC far field test, although it is no guarantee.

Watch Dr. Eric Bogatin demonstrate many bench top tests for radiated emissions in the on-demand webinar, Pre-compliance EMC Testing with a Real-time Oscilloscope.

Also see:

Unintentional Antennas in Electric Circuits

Pre-compliance EMC Testing Using a Real-time Oscilloscope