Editing Alternating current (section)

==== Waveguides ====
[[Waveguide (electromagnetism)|Waveguides]] are similar to coaxial cables, as both consist of tubes, with the biggest difference being that waveguides have no inner conductor. Waveguides can have any arbitrary cross section, but rectangular cross sections are the most common. Because waveguides do not have an inner conductor to carry a return current, waveguides cannot deliver energy by means of an [[electric current]], but rather by means of a ''guided'' [[electromagnetic field]]. Although [[Current density|surface currents]] do flow on the inner walls of the waveguides, those surface currents do not carry power. Power is carried by the guided electromagnetic fields. The surface currents are set up by the guided electromagnetic fields and have the effect of keeping the fields inside the waveguide and preventing leakage of the fields to the space outside the waveguide. Waveguides have dimensions comparable to the [[wavelength]] of the alternating current to be transmitted, so they are feasible only at microwave frequencies. In addition to this mechanical feasibility, [[electrical resistance]] of the non-ideal metals forming the walls of the waveguide causes [[dissipation]] of power (surface currents flowing on lossy [[electrical conductor|conductors]] dissipate power). At higher frequencies, the power lost to this dissipation becomes unacceptably large.