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Frequency of WPT
WPT must be high frequency in order to achieve high efficiencies. Typically, the range of a few kHz to MHz is used for near-field WPT technologies as inductive and capacitive WPT. For radiative WPT, frequencies higher than GHz are typical.
In general, lower frequencies correspond to higher (antenna) sizes and vice versa.
The Qi standard operates in the range 110 to 205 kHz.
The available frequencies in the radio spectrum, ranging from 3 Hz to 3 THz, are limited. Regulations in different countries restrict the free use of certain frequency bands. Moreover, electromagnetic interference can hinder the performance of other devices. As a result, certainly for experimental set-ups, frequencies in the ISM bands are often used. These bands are portions of the radio spectrum reserved internationally for industrial, scientific, and medical (ISM) purposes. With regard to near-field WPT, the bands around 6.78 MHz, 13.56 MHz and sometimes 27.12 MHz are most prevalent. For far-field WPT, the ISM bands around 2.45 GHz and 5.8 GHz are often employed.