V.S. Melnikova - Post-graduate Student, Saratov National Research State University named N.G. Chernyshevsky. E-mail: MelnikovaVeronica@yandex.ru O.V. Polischuk - Ph.D. (Phys.-Math.), Senior Research Scientist, Saratov branch of Kotelnikov IRE RAS V.V. Popov - Dr.Sc. (Phys. Math.), Head of Laboratory, Saratov branch of Kotelnikov IRE RAS; Professor, Saratov National Research State University named N.G. Chernyshevsky