  A wireless power transfer experiment suitable for senior physics
undergraduates that operates between 3 and 4 MHz is described and demonstrated
in detail. The apparatus 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. A matching
circuit was used to tune the impedance of the system of coupled resonators to
match the 50-$\ohm$ output impedance of the signal generator. A low-cost vector
network analyzer was used to characterize the system performance as the
distance between the pair of coils was changed. When the distance between the
transmitting and receiving coils was small, a double resonance emerged. The
frequency difference between the pair resonances was inversely proportional to
the distance between the coils. After the system was fully characterized, it
was operated with up to 20 W of incident power. Our measurements revealed that,
at weak coupling, the fraction of the incident power lost to radiation
increases linearly with the distance between the coils. Finally, we were able
to transmit enough power to dimly light a 60-W incandescent light bulb.
