1. The Antenna Company, High Tech Campus 29, 5656 AE Eindhoven, The Netherlands
2. Department of Electrical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
3. Department of Mathematics and Physics, Roma Tre University, Largo San Leonardo Murialdo 1, 00146 Rome, Italy
4. Mathematics Section, International Telematic University UniNettuno, Corso Vittorio Emanuele II, 39, 00186 Rome, Italy