Quasi-isotropic electrically small antennas for UHF-RFID passive tags based on 2-turns spiral resonators

Most of the current RFID passive tags in the UHF band are based on resonant dipoles, which are usually meandered in order to reduce dimensions. The dipole antennas provide single linear polarization, which involves the existence of blind spots in some regions of the radiation pattern (Brouwer, Proc. of KNAW, 11, 850–858, 1909) (e.g. the dipole antenna has a toroidal radiation pattern with two nulls in the direction of dipole axis). Thus, passive tags are not detected when the reader is placed in the direction of a blind spot. To overcome this drawback, some passive tags use specific ASICs with two fully independent, differential inputs enabling isotropic (or quasi-isotropic) reading patterns. However, two orthogonal antennas are required to avoid blind spots. Consequently, more complex tags with increased dimension are required.