In an word - DON'T!
Use a Collins coaxial balun (not to be confused with coiled coax choke).
This balun was first described in a Collins Radio publication in the '50's
and there have been recent descriptions in a CQ magazine article by Bill
Orr (Nov 1993), and a rather complete treatment of an entire series of coax
baluns based upon the Collins principal to match various load impedances
was published by George Badger, W6TC in Ham Radio (Feb/Mar 1980).
They are very economical to make, will survive any power you can afford to
deliver to them, and you don't have to worry about core saturation and
arcing when there's ice on your load. I have made versions from Teflon
coax for commercial applications which are capable of handling 25 kW :
RG-213, 9913, LMR-400 series of coax will more than handle any
amateur power level. All that's needed is about 17 feet of coax, two coax
connectors, a coax "T", some 3M electrical tape, and about an hour of work.
PS: If you are using a trapped tri-bander, can your traps handle the power
you intend to deliver to it? Especially when loaded with ice or when they
are wet? Be carefull here!
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