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Wireless Power Transfer via Radiowaves by Naoki Shinohara

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Chapter 3

Technologies of WPT

3.1. Introduction

As shown in Chapter 2, a power can be transmitted through radiowaves in accordance with the theory based on Maxwell’s equations and antenna theory and also through high frequencies based on electromagnetic theory and coupling theory. In addition, a frequency converter with high efficiency is required to convert DC or 50/60 Hz AC to high frequencies for transmitting wireless power, and high-efficiency rectifiers are needed to convert from high frequency back to DC or 50/60 Hz AC.

Especially for microwave power transfer (MPT), high-power amplifiers (HPAs) using semiconductors and a microwave tube are often used as a power amplifier and microwave generator. In general, the characteristics of an HPA using semiconductors and the microwave tube are as follows:

–HPA using semiconductors (GaAs, GaN, etc.) for MPT (Figure 3.1(a)):
- low power (less than several dozen watts using GaAs semiconductors and less than several hundred watts using GaN semiconductors);
- low voltage requirement (<5 V using GaAs semiconductors and 50 V using GaN semiconductors);
- efficiency (<80% in the 2 GHz band, <70% in the 5 GHz band);
- small size (with monolithic microwave integrated circuit (MMIC) technology);
- expensive (depending on mass production);
- suitable for a phased array (because of low power and small size).
– Microwave tube (magnetron, klystron, traveling wave tube (TWT), etc.) for MPT (Figure 3.1(b)):
- higher power (greater than a kilowatt); ...

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