Measurements of, and propagation models for the wide-band HF channel

Modelling of the ionospheric transfer function for HF radio propagation is a difficult task, since the nature of the ionosphere is very dynamic and sometimes stochastic. By using a frequency hopper as a channel probe, one can follow the dynamics of a wide-band channel. This gives a better means to interpret the channel's behaviour, than a narrow-band probe. Thus, one has a useful experimental base for designing a wide-band model for the deterministic part of the transfer function. The approach for the modelling work is to use a combination of theoretical considerations and measured conditions. The theoretical expressions used are derived from a sech/sup 2/-model of the electron density profile; see Lundborg et al. (1994), and Vogler and Hoffmeyer (1988). The state of the ionosphere, used as input in the models, is obtained with a chirp sounder, producing oblique ionograms every fifth minute. The authors see that different fading phenomena, observed with the frequency hopper, can be categorised simply by checking where the frequency hopping (FH) band is situated in the oblique ionogram. Hence, if the information on the state of the ionosphere is available (ionogram or prediction program), when modelling (or using) the channel one can to some degree, predict the fading in different frequency bands.<
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