It is important to understand why HF radars change frequency the way they do. This is how, in part, they control what area they are looking at. Typically, they may be able to control angle in Azimuth to direct a beam a specific direction. But they generally have no, or very little, control of angle in Elevation. So instead they use changing propagation conditions and frequency selection to control where their RF comes down onto the Earths surface or a desired piece of air space.
How do they know which frequency or frequency set to use at any given time? Most HF radars either have a dedicated ionsopheric test tool or mode, or they have access to such real time data. Some appear to have logic that tells them "this frequency range at this time of day, for these desired results", and you will find these radars jumping around, hitting multiple frequencies for short periods, until the arrive at a combination that works, then they may stay on that freq for a while.
Most frequency agile HF radars do not have defined frequency channels they use. That means you will not find them on specific frequencies. At best each specific type of radar will have specific frequency ranges that they may operate in. Combine that with typical propagation and typical target sets for each specific radar and you can start to get an idea of what frequency ranges to look for each radar in at specific times of day.
So, no real way to predict frequencies, other than in a very general way.
But, most radars are clearly identified on a waterfall or spectrum display. Once you learn what they each look like you can easily and quickly find them on wideband displays, allowing you to tune to each quickly after it changes freqs or as it moves to new frequency sets.
An important fact is that there is a LOT of variability. Different radars, even with the same mission sets, may act and react very differently. They may also use radically different techniques or waveforms, so that they can be extremely varied in appearance and sound.
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