Design of Chipless RFID Tags Based on Characteristic Mode Theory (CMT) The induced currents and consequently the scattered fields from chipless RFID tags can be analyzed in different ways. In singularity expansion method (SEM), the currents on the tag are expanded by a series of the complex natural resonances (CNRs) and frequency-independent natural currents and an entire-domain function including the early-time response. On the other hand, in eigen-mode expansion method (EEM), the currents are expanded versus the frequency-dependent eigen-modes of the structure, which can be obtained by solving the homogeneous differential equation governing the structure under consideration. In this paper, the theory of characteristic modes as a generalized eigen-mode expansion approach is efficiently employed for the analysis and design of the chipless RFID tags. Three important parameters usually considered in the design of multibit tags are the quality factors, resonant frequencies of the resonators (as a signature of the tag) and the radar cross-section (RCS) of the tag. A slotted-wavelength resonator is considered as a tag prototype in this paper. After a short theoretical description of SEM and CMT, the resonant and radiation modes of the tag are accurately investigated versus some design parameters. By studying the effects of various structural parameters of the tag on the aforementioned factors, some design rules are extracted which can be useful in the design and implementation of the tag and additionally the identification process.