  We describe efficient four-coil inductive power transfer (IPT) systems that
operate at 100 MHz. The magnetically-coupled transmitter and receiver were made
from electrically-small and high-Q loop-gap resonators (LGRs). In contrast to
the commonly-used helical and spiral resonators, the LGR design has the
distinct advantage that electric fields are strongly confined to the capacitive
gap of the resonator. With negligible fringing electric fields in the
surrounding space, the IPT system is immune to interference from nearby
dielectric objects, even when they are in close proximity to the transmitter
and/or receiver. We experimented with both cylindrical and split-toroidal LGR
geometries. Although both systems performed well under laboratory conditions,
the toroidal geometry has the additional advantage that the magnetic flux is
weak everywhere except within the bore of the LGR and in the space directly
between the transmitter and receiver. Furthermore, we show that the toroidal
LGR system can be operated efficiently at a fixed frequency for a wide range of
transmitter-receiver distances. The experimental results are complimented by
3-D finite-element simulations which were used to investigate the
electromagnetic field profiles and surface current density distributions.
Finally, we demonstrate the use of our IPT system at powers up to 32 W and
discuss possible applications.
