  Development of compact wireless power transfer (WPT) systems for charging
miniature randomly oriented electronic devices is quite a challenge.
Traditionally, WPT systems based on resonant magnetic coupling utilize
face-to-face aligned transmitter and receiver coils providing sufficient
efficiency at relatively large distances. However, with the presence of angular
receiver misalignment in a such system, the mutual coupling decreases resulting
in a low power transfer efficiency. Here we develop a compact WPT system for
wireless charging of miniature receivers vertically oriented with respect to
the transmitter. As a transmitter, we employ a butterfly coil that provides a
strong tangential component of the magnetic field. Thus, a vertically oriented
receiver located in the magnetic field can be charged wirelessly. We perform
numerical and experimental studies of the WPT system power transfer efficiency
as a function of the distance between the transmitter and the receiver. The
misalignment and rotation dependencies of power transfer efficiency are also
experimentally studied. We demonstrate the power transfer efficiency of 60 %
within transfer distance of 4 mm for a vertically oriented receiver with an
overall dimension of 20 mm X 14 mm at the frequency of 6.78 MHz.
