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28 DESIGN007 MAGAZINE I JULY 2023 Plane cavities in multilayer PCBs are essen- tially unterminated radial transmission lines. ey form a transmission line that propagates electromagnetic (EM) energy within a plane cavity emanating from a feed point within the plane and outward in all directions. Like all transmission lines, it will reflect if not ter- minated. is creates standing waves—ring- ing. e bigger the mismatch, the bigger the standing waves, and the more the impedance will be location-dependent. As frequency and edge rates continue to increase, the impact of intrinsic electrical characteristics becomes more pronounced. AC switching currents in the power/ground planes can be very large. Under these circumstances, a plane pair acts more like a radial transmission line rather than a distributed planar capacitor. A parallel plate capacitor (or a planar pair) has two conductors separated by a dielectric layer. Most of the electromagnetic energy in the structure is concentrated directly between the plates. However, some of the energy radi- ates into the area outside the plates. e elec- Standing Waves in Multilayer PCB Plane Cavities Beyond Design Feature Column by Barry Olney, IN-CIRCUIT DESIGN PTY LTD / AUSTRALIA