This pdf is quite useful:
http://members.aol.com/DXerCapeCod/z_transformers.pdf

It takes you through how to do the calculations for any torroid that you
purchase.

Large torroids and Wire Wrap wire (solid core) are quite effective and easy
to handle.

I found that Type 43 material with even with windings for 2 mhz and above
is still quite effective for MW. Effective enough that I get images on my
Tecsun PL-550 between 2 mhz and 3 mhz of distant MW broadcast stations that
disappear when I unplug the antenna.