Copied with permission from email@example.com where there is a thread on this interesting subject:
“Last night I observed Luxembourg effect for the very first time on MW frequencies.
I was monitoring Morocco on 596kHz at 0030utc as it was exceptional strong but as usual with a low level of modulation.
I could initially hear another source of audio mixing with the Arabic phone in conversation but there isn’t anyone else on 596kHz.
I then switched to 540kHz to hear the parallel Moroccan and to my surprise it sounded the same.
My first conclusion was that audio leaving the studio was being corrupted with another source of programming.
Then on closer listening on 596 (made possible by low modulation) with the audio turned up I heard English words. That was surprise.
Closer inspection revealed this to be audio from BBC Radio 4 – parallel with 198kHz.
So, in conclusion, it appears that the BBC LW signal was causing ionospheric cross modulation on both signals from Morocco.
73 Steve Whitt“
Mike Terry to Longwaveradiolistening iog (2022-02-01)