Tam/WB2TT wrote:

I can't remember the numbers, but there is a procedure in either the ARRL
Handbook, or antenna book for doing this. Basically, you have a short piece
of 50 Ohm coax, a short piece of 75 Ohm coax, and then an arbitrary length
of 50 Ohm transmission line.

You are thinking of a nonsynchronous balun described he

http://www.k1ttt.net/technote/matching.html#k1xx

HOWEVER, this only works for one band (i.e. a monobander...not a
log-periodic covering multiple bands). If the antenna is 75 ohms, why
not simply use 75 ohm coax to the shack and then a tuner to convert to
50 ohms? Seems much simpler and you can normally get free 75 ohm
hardline for the asking at your local cable company (i.e. reel ends).

73, Bill W4ZV