For ferrites commonly used in SWL baluns/ununs:
If you are using type 43 material in an RF transformer, performance will
fall off shortly after 10 Mhz. If the core is wound as a true balun/TLT
(Transmission Line Transformer), I have used it up to 30 Mhz.

Type 61 material is much more suitable for RF transformer from .2 Mhz to
100 Mhz. If wound as a true balun/TLT, it can be pressed into even
broader frequency service.

This page points out the above:
http://www.palomar-engineers.com/Fer...ite_cores.html

With iron:
Type 2, such as T106-2, when wound as an RF transformer is very
satisfactory to a bit beyond 10 Mhz. Wound as a true balun, I have used
it up to 30 Mhz. (it is really too lossy, for me, at 28 Mhz, 10m)

Type 6 material, such as T106-6, is good from 2-30 Mhz when used as RF
transformer (actually, I find the bandwidth to be larger in practice.)
If wound as a balun/unun/TLT, it is quite usable from MW to ~50 Mhz.
When wound as the 16:1/9:1 as I pointed out earlier, I find this to be
THE KILLER BALUN/UNUN (can be used interchangeably.)

This page points out the suitability of the material above:
http://www.palomar-engineers.com/Iro...on_powder.html

Regards,
JS