While not technically an antenna, the coil is the way inductively coupled tags communicate with the reader.
It is composed from loops of conductive material, ranging from half a dozen loops made with etched metal or conductive ink in HF tags to many hundred copper wire loops for LF tags.
The main characterisic of a coil is the size of area surrounded. With a bigger the area, the tag is able to draw more power from the reader increasing its range. For LF tags, a ferromagnetic material is often used as core, whith the purpose of reducing the number of loops required.
