[1] Univ Messina, Dept Elect Engn Ind Chem & Engn, I-98166 Messina, Italy
[2] Politecn Bari, Dept Elect & Informat Engn, I-70125 Bari, Italy
[3] Univ Roma Tre, Dept Engn, I-00146 Rome, Italy
[4] Ist Off Mat, CNR, IOM, Unita Perugia,Dipartimento Fis & Geol, I-06123 Perugia, Italy