EMC Question of the Week: April 27, 2017
Interference between an AM car radio (1.5 MHz) and an overhead power line is likely due to,
- conducted coupling
- electric field coupling
- magnetic field coupling
- radiation coupling
Answer
The correct answer is "b". Although the titles of EMC test standards give the impression that there are two EM coupling mechanisms, conducted and radiated; it is important for EMC engineers to recognize that there are four distinct EM coupling mechanisms. Identifying the correct coupling mechanism is an important first step when troubleshooting an EMC problem.
In the example above, we can rule out conducted coupling, because there are not at least two conducting connections between the source and the victim. We can also rule out radiation coupling, because the separation between the source and the victim is much less than a wavelength at the interference frequency. (At 1.5 MHz, the free space wavelength is 200 meters.)
The whip or shark-fin antennas on most cars pick up electric fields. They are not sensitive to magnetic fields. Therefore, any interference from an overhead power line must be due to electric field coupling.
We should note here that many hand-held radio receivers use multi-turn loop antennas to receive AM radio signals. These receivers pick up magnetic fields and are relatively insensitive to electric fields.
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