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In principle, the distance of wireless power transfer (WPT) via radio waves from a transmitting antenna to a receiving one can be extended if the medium of wave propagation is lossless, e.g., vacuum in space. Technically, the distance limitation of WPT via radio waves is dominated by the power required by users as well as the limitation of system size and efficiency in practical applications. It is easy to develop multiuser WPT systems using WPT via radio waves because they are electromagnetically uncoupled and no interference between circuit parameters at a transmitter occurs even with increasing number of users. Besides, since the beam efficiency between a transmitting antenna and a receiving one is not high, multiuser WPT systems are also suitable when WPT systems via radio waves are applied in far field.

In this chapter, the design methods of multi-hop wireless power transmission are described. First, the characteristics of multi-hop wireless power transmission are overviewed. Based on that, a design method based on bandpass filter (BPF) theory and a design method for realizing efficiency maximization in arbitrary hop are derived. Power efficiencies with the design theories presented here are evaluated in Section 4.6.