Single-hop F2 propagation above 30 MHz and over distances greater than 4000 km

During recent years of increased solar activity there has been an increase in the number of reports of communications at 50 MHz over distances exceeding 4000 km. This has led to discussion on the propagation modes and to suggestions that the standard textbook explanation fails to explain that such propagation can occur via single-hop F2 modes. It is pointed out that the standard textbook explanation based on parabolic layers predicts such modes. The most relevant ionospheric properties are briefly discussed, and some results obtained using quasi-parabolic segments (QPSs) to represent the ionosphere are presented. The QPS model provides analytical expressions for ray parameters for propagation in spherically stratified ionospheres. It is shown that the lower E and F1 layers can extend the range of F2 propagation beyond 4000 km