What factors determine the characteristic impedance of a coaxial transmission line?

Just like an open-wire line, the characteristic impedance of a coaxial transmission line is governed exclusively by its physical construction and the dielectric material separating the conductors. For coaxial cable, the key geometric factor is the ratio of the inside diameter of the outer shield to the outside diameter of the inner conductor. If the center conductor is made thicker while keeping the outer shield the same size, the characteristic impedance decreases. Conversely, increasing the size of the outer shield relative to the center conductor increases the impedance. Line length and signal frequency do not determine the characteristic impedance. A 50-ohm coaxial cable remains a 50-ohm cable regardless of whether it is 10 feet or 100 feet long, and regardless of the frequency of the radio waves traveling through it.