The MFJ-912 W9INN BALUN BOX is a broadband (1.8-30 MHz) toroidal balun. It provides a convenient method of feeding balanced line transmission lines remotely without actually bringing the line into the house.
The MFJ-912 is basically a 1:4 impedance transformer which converts an unbalanced output to a balanced output.
The impedance present at the balun where the balanced line is attached can be extremely high depending on the frequency of operation and the length of the transmission line. In light of this fact, use as large of coax as possible, at least RG-8/U or RG-8X/U. Keep the ends of the ceramic standoffs at least two inches away from any conducting surface.
Another thing about the coax is that almost all losses will be within it. The beauty of running ladder line is its extremely low loss. There will still be impedance mismatches in the coax and thus, high losses. Use as short a length of coax as possible, 50 feet or less. With 50 feet or less of RG-8/U or RG-8X/U, losses at the lower frequencies are negligible.