A transmission line is a pair of conductors which is used to transmit a signal and which is very long compared to the wavelength of the signal being sent. Communications engineers and power systems engineers both use the term transmission line, but they make different assumptions. To a communications engineer, it is a long pair of conductors over which a signal is sent. To a power systems engineer, it is a cable that is part of the power grid. The communications definition will be used in this text. The conductors of a transmission line may be a pair of parallel wires, they may be a waveguide formed by a pair of parallel plates, they may be a coax cable, or they may have another geometry. Coax cable is formed by a wire and cylindrical tube separated by an insulator, both with the same axis, so they are coaxial. For example, a coax cable connecting a transmitter operating at a frequency of $$f = 88 \text{ MHz}$$ on the first floor of a building and an antenna on the top of the tenth floor of the building is a transmission line because the length of the cable is long compared to the wavelength of $$\lambda = 3.4 m$$. As another example, a pair of wires connecting a transmitting circuit operating at $$f = 4 \text{ GHz}$$ on one end of a printed circuit board and an antenna on the other end 25 cm away is also a transmission line because the length of the wires is long compared to the wavelength of $$\lambda = 7.5 cm$$.