Fair-Rite 5943003801 MnZn Ferrite Toroid, 43 material, for EMI suppression between 20 and 250MHz, and wideband transformers up to 50MHz.  ui=800.  These are ideal for use as noise or common mode suppressors on cables to reduce interference on the HF bands, or constructing baluns.  These are recommended for use on HDMI cables where the cable connects to the computer video card to suppress noise in the 3 to 20MHz bands. 

For 100 Watts CW / FT8 or > 300 Watts SSB, It is recommended that two FT240-43 are stacked for use as a 49:1 Transformer for higher efficiency around 90% to reduce heating of the cores.  A single core is maybe 80% efficient and will run cool enough up to 80 Watts CW or FT8.  See End fed half wave antennas by Steve K1RF on how to make a 49:1 Transformer. 

Size: Diameter 61mm, Height 12.55mm Hole Size 36mm, color Grey, Fair-Rite PN# 5943003801

DIY 40m long 80-10m Band EFHW Antenna by the Utah Valley ARC.

See W1CG Low Power Balun Manual for more information on making a 4:1 Balun.

I highly recommend the excellent Youtube videos by Colin MM0OPX on tests that he has made with EFHW Transformers.