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The transmitted signal should stay within its transmission bandwidth even if distortion has been added earlier in the distribution chain, though I don't have an explanation and agree it's very puzzling.
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I had a bit of free time today to play with a larger (passive) antenna and this antenna works across the whole SW band. So I could compare BBC R4 against other stations.
I compared it against 1053kHz Talksport and both were received with about the same signal strength on the analyser (within a couple of dB). However, the Talksport transmission stays within its +/-5kHz bandwidth without any splashy IMD beyond this bandwidth. The BBC R4 transmission on 198kHz showed a fair bit of splashy IMD beyond +/- 5kHz today and I think it was slightly worse than my earlier video and the time pips had a bit more distortion on them today. So it is as if the signal quality has degraded a bit since it was 'fixed' last week.