=====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. 

Typically, higher frequencies correspond to smaller (antenna) sizes. 

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.

For near-field WPT, that uses inductors and capacitors, the efficiency of the system is related to the quality factor (Q-factor) of the circuit and the components. The higher the frequency, the higher the Q-factor, which means the lower the losses. Indeed, the Q-factor of a coil and a capacitor are linearly dependent on the frequency.