Interesting situation today - apparently quite common in this part of the world - a textbook 'hop'.
Perfect HF comms on 6 megs for position reports every 30 mins, and a bit of yak in between, then silence.
Call on the iridium to give report, and nothing heard on HF at all. HF radio check whilst in iridium comms, but still nothing. Then on the way back 'home', comms reestablished almost exactly at the same position.
A good read
here