A wireless power transfer experiment suitable for senior physics undergraduates is described and demonstrated in detail. The apparatus, which operates between 3 and 4 MHz, consists of a pair of identical resonant coils that can be moved relative to one another. A signal generator circuit is inductively coupled to the transmitting coil using a single loop of wire. Likewise, a single loop of wire couples the receiving coil to a load impedance. We use a matching circuit to tune the impedance of the system of coupled resonators to match the 50-Ω output impedance of the signal generator and a low-cost vector network analyzer to characterize the system performance as the distance between the pair of coils is changed. We find that when the distance between the transmitting and receiving coils is small, a double resonance emerges and that the frequency difference between the pair resonances is inversely proportional to the distance between the coils. After fully characterizing the system, it can be operated up to an incident power of 20 W. Our measurements reveal that, at weak coupling, the fraction of the incident power lost to radiation increases linearly with the distance between the coils. Finally, we demonstrate that the system is able to transmit enough power to dimly light a 60-W incandescent light bulb.

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