Because the coupling mechanisms for RF immunity are essentially the same as those for RF emissions, and because they are usually linear (unaffected by the amplitude of the coupled signal), the principle of reciprocity has evolved. In essence, it states that:

"reductions in coupling which reduce emissions over a particular frequency range will also improve immunity over that frequency range"

or, on the other hand,

"frequencies at which emissions are particularly troublesome will also be those at which immunity problems occur."

This concept relies on the observation that coupling mechanisms usually shown resonant behaviour which maximizes the coupling at certain frequencies. The reciprocity principle should not be accorded too much weight: actual EMC performance depends on the operation of both source and victim circuits as well as on the coupling between them, and these are rarely either reciprocal or linear. Understanding the principle can help in dealing with many coupling-related problems, though. This can be demonstrated in the case below:

Case Study

During the radiated emission tests of a large-size electric vehicle, it was found that a narrowband emission at 222MHz exceeded the limit line (a). It was found that the noise came from a camera that was fitted in the cabin of the vehicle. Multiple ferrite cores were used on the power leads of the camera, but the improvement was not significant enough to suppress the noise. The test also showed an “inconsistency,” as the same noise was measured a lot lower on some occasions (as shown in b). We accidentally discovered that the difference in the emission results was caused by the door of the vehicle. When the door was open, the noise emission was significantly less than when the door was closed.

By themselves, the vehicle cameras and their associated circuitry including the 20 cm long power leads were not an efficient radiator at the frequencies (and the harmonics) contained in the circuits. As shown in the picture below, when the door was closed, it was positioned near the camera area. The door was mainly glass, but the frame, and the wires embedded in the frame (connected to the door button, shown in green in the picture), together with the mechanical structure linking the door, was part of the metal enclosure and should be considered an EMC concern. Even though the door was not in physical contact with the camera, the parasitic capacitance and inductance coupled the RF energy of the camera onto it(near-field coupling), and the noise at 222 MHz was radiated very efficiently. At 222 MHz, a halfwave length wire is about half a meter. The structure shown in the picture can easily act as an efficient antenna.

The same structural issue in the door frame also caused immunity problems. This was evident during the portable immunity test, where test engineers used a portable device, such as a walkie-talkie, to assess the vehicle's resilience to external noise sources. The following video demonstrates that when a small power walkie-talkie is placed near the door and in use, it triggers the door to open and close.