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Wireless Power Transfer via Radiowaves by Naoki Shinohara

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Introduction

Wireless power transfer (WPT) is a promising technology based on electromagnetic theory and radiowave theory, representing the combined application of electrical and radio sciences. There are numerous WPT technologies, such as inductive coupling WPT (Figure I.1(a)) and resonant coupling WPT (Figure I.1(b)) as short distance WPT. In addition, WPT via radiowaves has been developed as a long-distance WPT technology, which includes focused beam microwave power transfer (MPT) (Figure I.1(c)) and energy harvesting from broadcasted radiowaves or diffused wireless power (Figure I.1(d)). Both inductive coupling WPT and resonant coupling WPT are based on electromagnetic theory. A transmitter and a receiver are electromagnetically coupled and power is wirelessly transmitted via an electric field, a magnetic field, or an electromagnetic field. Unlike the short-range technologies, WPT via radiowaves does not require coupling between the transmitter and the receiver, but it uses radiated electromagnetic waves. WPT via radiowaves requires higher frequencies, such as microwaves, to focus on the wireless power effectively. The general characteristics of various WPT technologies are described in Table I.1. The primary difference between inductive and resonant coupling is the presence of non-resonance and resonance, respectively. In the case of short distances, inductive or resonant coupling technologies using coils are effective, but the coupling distance is limited by coupling theory ...

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