These are windings made on plastic or other dielectric frames with copper, aluminium wire or printed conductors. Since such coils don’t have a core, this allows them to be used as emitters of the EMI interference (to complicate the operation of eavesdropping devices or mobile or other communications), as e.m. pulse rifles, and as the basis of the electromagnetic locks and special irradiators for medical purposes.
Most often, they are made turnkey so they can be of different shapes and designs, depending on the type of application. As a rule, they are used together with the high-frequency generators.