The problems of short-wave radio service in the region in which propagation modes are changing

The coverage area of a short-wave radio transmitter is considered under the assumption that interferences by other transmitters can be disregarded. It is found that this area is limited at large distances by propagation loss, while near the transmitter the skip zone has to be taken into account. However, observations of the received signal show that unsufficiently covered regions can also exist between these limits. Possible reasons for the occurrence of such regions are explored, taking into account the concept of the reliability of an ionospheric connection. If the operational frequency is high enough, zones of silence are found in the areas between the first and the second mode, and the second and the third mode. It is concluded that it is generally impossible to cover an area involving large distances without any gaps by using a single frequency. Such an objective requires an employment of additional frequencies.