A.M. Shihabudinov - Ph.D. (Phys.-Math.), Research Scientist, Saratov branch of Kotelnikov IRE RAS. E-mail: alex-sheih@ya.ru B.D. Zaitsev - Dr.Sc. (Phys.-Math.), Head of Laboratory, Saratov branch of Kotelnikov IRE RAS I.A. Borodina - Ph.D. (Phys.-Math.), Senior Research Scientist, Saratov branch of Kotelnikov IRE RAS A.A. Teplykh - Ph.D. (Phys.-Math.), Senior Research Scientist, Saratov branch of Kotelnikov IRE RAS I.E. Kuznetsova - Ph.D. (Phys.-Math.), Leading Research Scientist, Saratov branch of Kotelnikov IRE RAS