Core Material Selection Toroid cores are used for this construction. Some experimentation determined that a permeability of 125u is better for 1 to 60 MHz 4:1 BALUNs. The 125 u is equal to 850 u for 1 to 30 MHz 1:1 BALUNs. Ferrite cores can permanently change their permeability after being subjected to relatively high power so select the largest core which will handily fit into your enclosure. For high power (~1KW) 4:1 BALUN 1 to 30 MHz use dual 60mm 125 u core (60mm OD, 35mm ID, 10mm HT). The best 1:1 BALUN 1 to 60 MHz use dual 38mm 850u cores (38mm OD, 23mm ID, 10mm HT) with 13 turns (24 single turns) #14 AWG per core. For 100W BALUNs use #20 AWG wire.