Dennis,
If you had been able to talk to a WW2 RAF Signals Officer he would have tried to convince you that H/F W/T (High Frequency, Wireless Telegraphy – i.e. morse) frequency selection was a science. In actual fact (which he, and all the operators (both airborne, and on the ground) all knew) it was, in those days largely an art-form!! (Not a lot has changed!!!).
To pick the right frequency for long-distance morse communication depends on distance, time of day, time of year, right aerial, and a whole host of other (then unforcastable) variables! Any ground station might not pick up an a/c only a few hundred miles away – but could hear an a/c, or another ground station, thousands of miles away (atmospherics permitting!). During the Falklands War, I could often not pick up directed Met H/F broadcasts from UK to Ascension (3650nm), but could hear Darwin, Oz,(8000nm away!).
HTH
Peter Davies