B.D. Zaitsev - Dr.Sc. (Phys.-Math.), Head of Laboratory, Saratov branch of Kotelnikov IRE RAS A.A. Teplykh - Ph.D. (Phys.-Math.), Senior Research Scientist, Saratov branch of Kotelnikov IRE RAS. Е-mail: teplykhaa@mail.ru А.М. Shikhabudinov - Ph.D. (Phys.-Math.), Research Scientist, Saratov branch of Kotelnikov IRE RAS