Features of Superlong-Distance and Round-the-World Propagation of HF Waves

We present the results of the experimental studies of the features of superlong-distance and round-the-world propagation of the HF waves in the radio lines with different orientation and length, which were obtained in 2012–2014 using a new method of oblique ionospheric sounding. The frequency–time travel intervals of the direct round-the-world signals, their amplitude-frequency and angular-frequency characteristics are determined. The mechanism of propagation and transformation of the round-the-world signals due to the radio-wave refraction by the transverse electron-density gradients in the region of approach of two optimal paths passing via the transmitter and receiver so that each path forms the smallest angle with the terminator. It is shown that the proposed mechanism is in good agreement with the experimentally observed variation of the azimuth of the direct round-the-world signal on the Cyprus–Rostov-on-Don path and on the Alice Springs (Australia)–Rostov-on-Don path in the absence of variation of the direct round-the-world signal azimuth. For the superlong-distance propagation of the HF waves on the Virginia (USA)–Yoshkar-Ola and Puerto Rico–Yoshkar-Ola (the distances about 8000–10000 km) paths, the best propagation conditions are observed when the entire path is in the illuminated ionosphere near the terminator boundary making a small angle of 10°–25° with the terminator.