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24 DESIGN007 MAGAZINE I FEBRUARY 2023 Electric and magnetic fields, which are used for the transport of energy, are equal to zero on the metal surface of the waveguide. erefore, these fields are confined to the waveguide's internal space, which minimizes losses. With- out the physical constraint of a waveguide, wave intensities decrease according to the inverse square law as they expand uniformly in all directions. Waveguides act like conduits for high frequency displacement currents. All transmission lines function as conduits of elec- tromagnetic energy when transporting pulses or high frequency waves. Wireless Power Most of us have Qi wireless chargers for our smart devices—who could live without them? e big advantage to using these chargers is that you do not have to constantly plug in cables to charge your phone. No wires—no current—but 15W of power. Qi wireless charging uses resonant inductive coupling between the sender (the charging sta- tion) and the receiver (the mobile device) as in Figure 2. Figure 3 shows the Qi receiver coil in an iPhone 12. e two coils act as a trans- former when a compatible device is placed on a charging station. Transformers A transformer works by electromagnetic induction (or mutual inductance). is occurs when two electrically isolated coils are in close proximity, such that one's magnetic field cou- ples to the other. When an alternating cur- rent is applied to the primary coil, a fluctuat- ing magnetic field is generated, which causes electromotive force in the secondary coil. is varying electric field creates displacement current in the secondary winding. Adding an iron core to the transformer improves the efficiency by directing the electromagnetic field so that it couples directly into the sec- Figure 2: Qi wireless charging simulation (simulated in Ansys Maxwell). Figure 3: Apple iPhone 12.