Andy

I found the same thing on topband and was puzzled so I did some tests. Only
below 500kHz did the particular grade of (quite thick) litz wire exhibit an
improvement.
As for why... I don't know, but the Fine Wire Company, which makes the stuff
has tables of spec for Litz and you have to use the right grade for the
frequency in use, presumably to match the skin effect to the strand
diameter.

Interesting stuff.

Dave G3YXM.