This reminds me of my time in submarines some 40-50 years ago when from time to time we would have difficulty in clearing our 'check reports' (during non-covert ops these were mandatory signals we had to transmit during fixed time-slots to confirm to MOD at Northwood that we hadn't sunk). The South China Sea, North Atlantic and Southern Arctic were (if I remember correctly) particularly difficult areas and more often than not we could only clear the signal via say, Cape Town or some other equally distant station. This was quite remarkable if you consider that frequently we would transmit these signals while submerged at periscope depth with the radio mast only a few feet above the surface. I seem to remember references made to 'sky wave' effect. I was just a simple navigator and while I was quite used to using the equipment, the science behind radio communications was all very mysterious to me; somewhat of a 'black art'. Those were the days!
Frank