Influence of the interaction between antenna currents and return currents on the coupling between digital interfaces and on-board antennas

Due to the integration of wireless modules and on-board antennas at printed circuit board (PCB) level, design engineers are faced with even more difficult challenges to avoid intra-system electromagnetic interference issues. When starting to design a printed circuit board, the first rule is to partition the printed circuit board such that noisy components are placed physically far away from sensitive components. This rule relies on the phenomenon that at moderately high frequencies, return currents tend to stay in proximity of and close to their signal currents. Unfortunately, on-board antennas most often induce antenna currents which are distributed over a large part of the PCB´s power/ground planes. As a result, significant interference might even happen with circuits which are placed physically far away from the antenna´s location. During the whole design process, the engineer has to make sure that the overlap area between the antenna currents and the return currents paths of the critical circuits is as minimal as possible. This is achieved by properly placing circuits, keeping track of the antenna current distribution, and by avoiding all return path discontinuities. “Intuitive” solutions as putting a slot around the critical circuits might actually lead to higher electromagnetic interference levels. Hence, full-wave simulations which allow for visualizing all current distributions and their interactions, are an important asset in aiding the design process. The simulation process becomes a key element in a judicial component placement to minimize the interaction between antenna currents and return currents in PCB designs.