Problem ot the energy transfer for radio paths near single-hop limiting distance for low solar activity

Formation the feature, in a form of deep trough, in frequency dependence of the wave field strength for single-hop paths with distances near classical limiting distance 3000 km at low level of solar activity was considered. Model calculations within the framework of the extended global ionospheric IRI model show high probability for appearing such a situation in the local daytime with a developed regular E-layer of the ionosphere. Some experimental results in multifrequency radio sounding of the ionosphere with a registration of the deep trough in frequency dependence of signal-to-noise ratio (SNR) were analyzed. It is shown that the IRI model, in principle, makes it possible to reproduce this peculiarity in the wave field energy parameters, but in some cases of experimental data, to a large extent, is able to provide only a qualitative description of this effect. Possible reasons for the quantitative discrepancy between experimental and model results are discussed.