THE INVESTIGATION OF LONG-DISTANCE HF PROPAGATION ON THE BASIS OF A CHIRP SOUNDER

Experimental data on round-the-world HF radio signals near the termina tor are given. The critical frequency of the ionspheric waveguide is f ound to be f(c)-is-similar-to-16-17 MHz. At frequencies f-is-less-than -f(c) the group delay has a negative dispersion t(g) = delta-tau/delta -f-is-similar-or-equal-to -100 mu-s/MHz for frequencies greater-than-f (c). Ray-tracing calculations are carried out. It is found that the lo w frequency branch of the round-the-world signals (f-is-less-than-f(c) ) by ricochet and hop modes. Experiments on the waveguide modes escapi ng from the ionspheric channel due to field-aligned scattering by arti ficial ionspheric turbulence are carried out. The conditions for trapp ing of radio waves in the ionospheric waveguide are investigated. It i s shown that if the gradient of the critical frequency f(0)F2 is less than minus 2x10(-2) MHz/100 km radio wave trapping takes place in the ionospheric waveguide at frequencies exceeding by 1-2 MHz the maximum observed frequency of the hop mode. The frequency time characteristics of the mode and the geophysical conditions for the effective control of radio waves escaping from the waveguide are defined.