Mathematical Representation of Scattered Fields from Chipless RFID Tags

Abstract

As previously mentioned, a chipless RFID system is comprised of three basic components: reader, antenna, and chipless tag. The antenna illuminates the reader area and induces currents on the metallic tags. The induced currents re-radiate the scattered fields, which will be processed in the reader for decoding the IDs of the tags. The scattering phenomena is a sophisticated process, which can be described in a simple mathematical model. This mathematical model provides us insight of the electromagnetic behavior of the structure, which is useful in the design process of chipless RFID tags. The induced currents on the tag structure can be expanded in different ways. One method is to expand the induced currents versus the singularity poles of the tag, which is the basis of the singularity expansion method (SEM). In such a representation, the solution is expressed as a collection of poles, branch cuts, and an entire function in the complex frequency plane. In this chapter, after a comprehensive study of the SEM, the wavefront representation of the SEM is presented to describe the scattering mechanisms in the early-time and late-time modes. Altes’ model is employed to describe the early-time response using the impulse responses of the scattering centers of the scatterer. Subsequently, the equivalent circuit of the scatterer is introduced based on the SEM representation of the fields. As an example, the current modes and associated radiated fields from a dipole antenna are studied.