Last evening I was tuning up the HF band with my SDR. Lots of stations to listen to. As usual I hear 4840 kHz good and strong. Later I ran across the Ampegon stunning signal from WBCQ on 9330 kHz. Later I found a wavering, but often moderately strong AM signal (much stronger than any other ham transmission) at 14170 kHz with both stations' audio mixed together nearly perfectly. No surprise 4840 + 9330 = 14170;. A friend made me aware of the Luxemburg–Gorky effect... https://en.wikipedia.org/wiki/Luxemburg–Gorky_effect ...but that suggests you hear the mixing on one of the two original frequencies rather than the mixing sum frequency. Anyone have any idea what might be causing this effect? I haven't ruled out my receiving setup either, but others are invited to look for this effect when 9330 fires up.