Here’s One for All You Propagation Gurus

This evening, just after dark, I called CQ on 40m CW. On the second call, VE3QO, in Ottawa, ON replied to my call. On that first transmission, he was at least 10 dB over S9, and that’s saying something because the S-meter in my IC-746PRO rarely reads anything over S9. Unfortunately, on the next go-around, VE3QO was a lot weaker, and on his third transmission he was nearly unreadable.

This is not the first time that I’ve noticed this phenomenon. It often occurs just after it’s gotten dark. In fact, I can often predict that this will happen by how strong a station is on his or her first transmission. The more out of the ordinary the signal strength, the more likely it is that the station will also disappear quickly.

My question is what propagation mechanism is causing this behavior? Is it perhaps the combining of the F1 and F2 layers? If that’s the case, why is the calling station so unusually strong on the first transmission?

Comments

  1. Martin AA6E says:
  2. Dan KB6NU says:

    I don’t think the selection effect accounts for this phenomenon because the signals are much stronger than usual. The other idea has merit, though, and I’ll have to think about this more.

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