Why is 50ohm sometimes used as a differential impedance?
The simplest balun is bifilar wire wound on ferrite. This gives a 1:1 impedance transformation from input to output, so 50&nbsp;&Omega; in, 50&nbsp;&Omega; out. It's not only the simplest, but best performance of balun topologies.

Get the balun close enough to the mixer, less than 1/10 of a wavelength, and the impedance error of more practical trace widths produces negligible signal loss. Of course at 5GHz that's going to have to be pretty darn close.