Design of License Plate RFID Tag Antenna Using Characteristic Mode Pattern Synthesis

A novel design of the passive license plate radio frequency identification tag antenna in UHF band based on the characteristic mode theory (CMT) is proposed in this paper. The license plate is operated as a tag antenna since it is conductor (made of aluminum). A desired up-tilted pattern beam is achieved successfully by using a direct CMT-based radiation pattern synthesis procedure. The pattern can be expanded as a superposition of some specific characteristic modes pattern, which is identified according to characteristic mode analysis of the plate. Four coupling slots are exploited to excite the selected modes current distribution. The mode weighting coefficients can be tuned to form the desired pattern beam by changing the relative phase between the coupling slots. The feed structure is carefully designed without damaging the layout of characters and numbers on the license plate. The synthesized radiation pattern corresponds with the pattern simulated by HFSS. The measured radiation pattern proves that the proposed design is reliable and applicable. The measured read range is about 20 m, which can satisfy the actual requirements.