1. Department of Physics, University of Maryland College Park, MD 20742-4111, USA
2. Engineering Research Institute, “Ventspils International Radio Astronomy Centre”, Ventspils University of Applied Sciences, Inzenieru 101, LV-3601, Ventspils, Latvia
3. Department of Physics and Methods of Teaching Physics, Berdyansk State Pedagogical University, 71100 Berdyansk, Ukraine
4. Institute of Solid State Physics, University of Latvia, Kengaraga iela 8, Latgales priekspilsēta, Riga, LV-1063, Latvia