flat coil
A flat coil is a flat air core coil consisting of one or two wires (bifilar) wound spirally in the plane. The coil wire can be wound onto a bobbin, but it can also be laminated as a printed circuit on a printed circuit board.
Flat coils do not have a coil core, they behave like normal coils and can be incorporated into electronic circuits. Flat coils are now rarely used in RF technology and audio technology, but there are new and interesting applications for this type of coil as frame coils in RFID cards, as discrete flat coils in wireless power transmission according to the Qi standard, and as printed flat coils in circuit boards of switching power supplies. The latter have an attached coil core to reduce dispersion.