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.