Abstract
Radiation characteristics including the radiation efficiency have been obtained for small spherical surface antennas by using the spherical wave expansion. Analytical expressions of the radiation efficiency revealed its theoretical limit [1] . Spherical helix antennas are proposed to approach the upper bound of the radiation efficiency. This antenna is composed of helical wires whose direction is along the optimized surface current of the spherical surface antenna. It is known that the radiation efficiency of a spherical helix antenna is always smaller than that of a spherical surface antenna. A method to predict the reduction of the radiation efficiency remains unclear. Here the radiation efficiency of spherical helix antennas is obtained analytically by modifying a result of the spherical surface antenna.
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