P. F. Denisenko - Dr.Sc. (Phys.-Math.), Professor, Head of Radio Physics and Space Researches Department of Scientific Research Institute of Physics, Southern Federal University. E-mail: denis@ip.sfedu.ru
A. I. Skazik - Ph.D. (Phys.-Math.), Associate Professor, Head of the Space Plasma Electrodynamics Laboratory of Scientific Research Institute of Physics, Southern Federal University. E-mail: askazik@mail.ru

For oblique ionospheric sounding by HF radio waves in about 10% of cases at the highest applicable frequency observed additional increase in radiation is not associated with a focus on the dead zone boundary. The possibility of explaining for the effect by influence of regular horizontal gradients of electron density due to the different geographical location of transmitting and receiving devices has been investigated on basis of the assessments of the energy loss of waves of extraordinary polarization for the route Moscow - Rostov (range of 950 km, the frequency of 9,996 MHz). It has been found that the horizontal gradients are not the determining factor leading to further increased radiation near the dead zone boundary. A possible reason of this effect can be middle scale traveling ionospheric disturbances, causing additional focus radiation.