The characteristic impedance of an open-wire transmission line depends, in part, on the diameter of its conductors. What other dimension determines its characteristic impedance?

The characteristic impedance of a parallel-conductor or open-wire transmission line is determined by two main physical dimensions: the diameter of the conductors and the center-to-center spacing between them. Increasing the distance between the two parallel wires increases the characteristic impedance, while increasing the diameter of the wires decreases it. The surrounding insulating material (usually air for open-wire line) also plays a critical role. Other factors listed, such as the length of the line, its height above ground, or its distance from metal structures, might affect the tuning or balance of the overall antenna system, but they do not change the fundamental characteristic impedance built into the transmission line itself.