5. Conclusions

The optimization of an antenna array pattern when the electric field r-decay factor effect cannot be neglected has been described, with particular emphasis on the context of DSRC systems. In fact, in order to maximize the available transaction time of ETC for future MLFF, it has been shown that particular attention has to be dedicated to the antenna array beam pattern synthesis on the road surface (or in a parallel surface plane). The generic optimization problem of a beam pattern defined on a planar surface has been introduced with the concept of coverage area. The coverage area is a bounded portion of the space in which the communication between RSU and OBU has to be guaranteed. After that, an optimization algorithm for specific coverage area geometries has been derived and solved through linear programming, highlighting the difficulties in achieving the synthesis due to the rdecay factor effect. Then, the design of the antenna array for maximizing the coverage area length in the specific RFID-based ETC case by employing a simple iterative method has been described. A 4 4 planar antenna array for UHF EPC Gen2 standard has been manufactured and employed as reader antenna during a measurement procedure which has demonstrated the validity of the proposed methodology.

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