When circuits are represented on schematics, we often indicate the return path as a signal ground or signal return with a ground symbol rather than indicating the complete return path with a solid line back to the source. If we trace the complete path the current would travel in such a circuit from the source to the load and back to the source, a loop is formed when the signal's forward path and the return path are combined in such a manner that the surface area enclosed by the loop has a finite value. In general, by increasing or decreasing the physical distance between the forward path and the return path, the loop area is increased or decreased respectively if all other circuit dimensions remain unchanged. All electrical current must return to its source thus forming a closed loop originating from the source and ending at the source. 

So what's so bad about loops? Nothing, They are a necessary aspect of all functioning circuits. That said, loops can act like antennas, as we will soon find out. Loop antennas can pick up unwanted signals or radiate unwanted energy that may interfere with other nearby circuits. This is usually bad news from an EMC perspective. 

Theoretically, when a loop is in the presence of a magnetic field, magnetic flux is produced in the loop. The rate of change of the flux induces a voltage in the loop that is directly proportional to the frequency of the changing magnetic field. The amount of magnetic flux in the loop can be approximated by product of the flux density and the area bounded by the loop.  As the loop area is increased, the total flux coupling the loop then increases thereby increasing the induced voltage in the loop for any given frequency. Note also that the induced voltage is also a function of frequency. As the frequency is increased, so does the induced voltage in the loop.

The voltage drop across any element in the loop depends largely on the induced voltage, the loop inductance and the resistance in the circuit. By increasing the loop area, the loop inductance is also increased. If the source or load resistance is high, relative to the loop's inductive reactance, then we can expect more of the magnetically induced voltage across the source and load resistance. On the other hand, if the resistance is low relative to the loop's inductive reactance, then most of the induced voltage will appear across the inductance. This makes high impedance circuits much more susceptible to picking up EMI due to magnetic coupling than low impedance circuits.

We've just discussed how loops can pick up unwanted signals, but what about their ability to radiate unwanted signals? For electrically small loops with dimensions much less than a wavelength, it can be shown that the radiated electric field can be estimated to be proportional to the driving voltage, frequency and the loop area for any given distance from the loop. The dominant mode of radiation may be either dipole mode or loop mode but generally speaking, radiation increases when the loop area increases. On printed circuit boards enclosed within a box, the radiated electric field can couple to nearby larger structures (e.g. heat sinks, cables) that can act as efficient antenna systems even though the radiated electrical field measured near the loop may actually be very small and well within emission standards. 
 

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EMC Tip 13

Last revised Sept 28, 2006