Abstract
This paper reviews and extends knowledge of the temporal and spatial characteristics of HF signals propagated via the ionosphere. The characteristics of HF signals are considered with reference to the problem of predicting signal strength on a group of assigned communications frequencies, using a channel sounding system. The particular application of sounding discussed is to provide predictions applicable (i) for many minutes (ii) for a communications path separated from the channel sounding path and (iii) for a communications path in a direction opposite to the channel sounding path. Studies are reported which showed that movements of small and intermediate sizes of irregularities resulted in non-stationary HF signal statistics. They also resulted in differences in averaged signal strength at receiving stations separated by 2 to 32 km. For 22 to 32 km separations, the differences were 4 to 5 db (for 68% of sample periods), when signal levels were averaged for eight minutes.
Other studies are reported which showed that ionospheric non-reciprocity resulted in unequal levels of signals travelling in opposite directions between linear antennas. The path non-reciprocity which arises when polarization fading was present was appreciably reduced by averaging signal levels for eight minutes.
It is concluded that inaccuracies in quantitative signal strength prediction can arise due to the effects of ionospheric irregularities and path non-reciprocity. These inaccuracies can be appreciably reduced by sampling and averaging signal strength measurements for at least eight minutes. Additional communications power margins must also be provided to allow for the remaining uncertainties in signal strength during the time of message transmission.
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References
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Jull, G.W. (1968). HF Spatial and Temporal Propagation Characteristics and Sounding Assisted Communications. In: Folkestad, K. (eds) Ionospheric Radio Communications. Springer, Boston, MA. https://doi.org/10.1007/978-1-4899-5511-1_17
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DOI: https://doi.org/10.1007/978-1-4899-5511-1_17
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