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16 DESIGN007 MAGAZINE I MARCH 2023 When two (or more) electromagnetic fields overlap or meet, they add vectorially at each point in space. Fields have direction and polar- ity. At any point in space, there can be only one field, so at some spatial points, they will cancel each other, and at others, they will re-enforce each other. James Clerk Maxwell described electromagnetic fields as being linear. Linear- ity implies superposition, meaning that the fields do not merge with each other but rather add vectorially—distorting the signal. is applies to both static (DC) and time-varying (AC) fields. When an electromagnetic field propagates along a transmission line, there are electric field lines between the signal and return paths and rings of magnetic field lines around the sig- nal and return path conductors. ese fields are The Interaction of Electromagnetic Fields not confined to the immediate space between the signal and return paths but rather, fringe fields spread out into the surrounding volume. In a stripline (inner) configuration (Figure 1a), the electromagnetic field propagates between the planes, and the return displacement current flows in close proximity (mainly) to the nearest plane. As the frequency increases, the current is forced into the outer surface of the copper, due to the skin effect, dramatically increasing loss. Figure 1b shows the electromagnetic field in a microstrip (outer layer) configuration. Electric fields terminate when they come into contact with a solid plane, whereas magnetic fields tend to radiate into the dielectric or air above or below the PCB. However, stripline (embedded signals) are rather confined by the adjacent planes and only emit radiation on the Beyond Design by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA with Special Advisor Rick Hartley, R HARTLEY ENTERPRISES Figure 1: a) EM fields in stripline configuration.; b) Microstrip configuration. (Source: Simboer & QuickField) a) b)

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