Hi Stefan,
I never tried #43 core on 6m band, so its difficult to say. I measured efficiency of different winding styles end even KN5L method shows rapid drop of efficiency above 30MHz. So even if SWR curve might look good efficiency could be very low. You could be heating #43 core rather then radiating. However #52 core has higher frequency range but this is at the cost of poorer performance at low bands (low permeability core). I done 49:1 transformer using FT140-52 and it was working ok from 10MHz upwards, however I didn't check 6m band.?
I will dig out this core and try to test in a week or so.
73 Marek
