Hi Johan ...

I don't know how much better Litz can do in your case but  the winding
length is also important for efficiency. If I remember correctly, best

efficiency

(power transfer) is achieved when the winding covers the entire length
of the ferrite rod i.e. the coil uses as much length as possible of the

concentrated

field . Unfortunately, this does not coincide with maximum coil Q.

Yes. Well remembered. My first try with the 20cm rod has more or less the
same turns amount but covering half of the rod lenght. Then I read somewhere
about covering all rod lenght to improve efficiency so I changed from a
thinner wire to this 0.5mm diameter one. Really improved.

Right after I wrote the messages asking about litz wire, instead of remove
some turns to change the freq covereage I spreaded the existing coil. The
freq covereage changed and the signals increased, I can say something
between 5 and 10dB. I´ll check later using the same SLM I used to generate a
6 frequency table with uV readings ...

Thanks on the comments.

73
Marcus, PY3CRX/PY2PLL
GG66rf
http://py.qsl.br
Active from "DC" to 24GHz