Optimum antenna coil structure for inductive powering of passive rfid tags

Radio frequency inductive links are becoming extensively used in wireless powering of embedded devices such as RFID systems and sensors. The design of RF coils for such links is often empirical and non optimal. Based on loop antenna theory we present a novel analytical model for near field inductive coupling incorporating misalignment of the RF coil system. Formulae are derived for the magnetic field at the tag coil when it is laterally and angularly misaligned from the reader and a new power transfer function presented. For the first time a near-field power transfer formula is suggested incorporating coil characteristics and misalignment factors. This novel power transfer function allows a comparison between different coil structures such as short solenoids, with air or ferromagnetic core, planar and printed spirals with respect to power delivered at the tag and its relative position to the reader. It is also possible to define the maximum degree of misalignment permissible in a given application. This analysis allows a formal design procedure to be established in order to identify the optimum coil geometry for RFID tags with respect to read range, angular, lateral displacement and operating frequency of the system.