Which of the following factors determine the characteristic impedance of a parallel conductor feed line?
Why Characteristic impedance is set by the line's geometry and the insulating material between the conductors, not by how the line is used. For open-wire (parallel conductor) line, Z0 is approximately 276 x log(D/r) where D is the center-to-center spacing and r is the conductor radius (the dielectric constant of the insulation also factors in). Spread the wires farther apart and Z0 rises; use fatter conductors and Z0 falls.
Watch out Length and frequency are tempting because they do affect loss and the impedance you measure at the input end when the line is mismatched, but the characteristic impedance itself stays the same whether the line is 3 feet or 300 feet long.
Z0 is built in at the factory: spacing and wire size only. Frequency and length change loss, not Z0.