1. George Green Institute for Electromagnetics Research, University of Nottingham, Nottingham, U.K.
2. Department of Physics, Loughborough University, Loughborough, U.K.
3. School of Mathematical Sciences and the George Green Institute for Electromagnetics Research, University of Nottingham, Nottingham, U.K.
4. Department of Industrial and Applied Mathematics, Delft University of Technology, Delft, The Netherlands