Yu. K. Kalinin

The variability of the ionosphere is characterized by growing critical frequency of F2 area while moving towards the equator from the polar regions. At different times drop of critical frequency can reach magnitude of ~ 5%/th.km. In these circumstances, the value of the MUF-F2-4000 at mid-latitudes when radiating over azimuths 0°,180° may vary up to 20%.The article presents the estimates of MUF-F2-4000 at a point distant from the emitter at 2 thousand km. north (azimuth 0°) and at a point distant from the emitter at 2 thousand km. south (azimuth180°). A similar difference of MUF-F2-4000 occurs in the other hemisphere. It is known that for ionospheric radio paths longer than 6000 km value of MUF-F2-4000 is defined as the lesser of the two values of this value, at points inside the track distant at 2000 km from its endings. So, if the corresponding points are located on either sides of the polar areas the size of MUF-F2-4000 on the reverse route significantly higher than on the direct.It is shown by calculations, and it is confirmed by experiment. There are two effects that are significant, the first - is the waveguide passing of radio wave packets over the absorbing region of the polar ionosphere.In this case the leap modes undergo additional absorption up to 40 dB, which makes it practically impossible to transfer information. The second - operation at frequencies exceeding the local MUF-F2-4000 occurs at the short waves range,less loaded with interference from other radio-electronic devices.It is also a favorable factor for the transmission of information.