WPT  is a term for the wireless transmission of electrical energy. The efficiency depends on the distance. In this case it is only possible over a relatively short distance (in most applications: a few cm to a few dm), because in free space electrical energy has the tendency to spread in all directions. The airborne transmission is only possible with alternating fields. In all commercial applications the transmission frequency is higher than that of the source (direct current, DC, or, AC frequency). The availability of converters with high efficiency (up to 98%), both for  DC–AC  and for AC–AC, makes  WPT increasingly attractive.