Editing Alternating current (section)

=== Techniques for reducing radiation loss ===
As written above, an alternating current is made of [[electric charge]] under periodic [[acceleration]], which causes [[electromagnetic radiation|radiation]] of [[electromagnetic waves]]. Energy that is radiated is lost. Depending on the frequency, different techniques are used to minimize the loss due to radiation.

==== Twisted pairs ====
At frequencies up to about 1&nbsp;GHz, pairs of wires are twisted together in a cable, forming a [[twisted pair]]. This reduces losses from [[electromagnetic radiation]] and [[inductive coupling]]. A twisted pair must be used with a [[Balanced line|balanced]] signaling system so that the two wires carry equal but opposite currents. Each wire in a twisted pair radiates a signal, but it is effectively canceled by radiation from the other wire, resulting in almost no radiation loss.

==== Coaxial cables ====
[[Coaxial cable]]s are commonly used at [[Audio frequency|audio frequencies]] and above for convenience. A coaxial cable has a conductive wire inside a conductive tube, separated by a [[dielectric]] layer. The current flowing on the surface of the inner conductor is equal and opposite to the current flowing on the inner surface of the outer tube. The electromagnetic field is thus completely contained within the tube, and (ideally) no energy is lost to radiation or coupling outside the tube. Coaxial cables have acceptably small losses for frequencies up to about 5&nbsp;GHz. For [[microwave]] frequencies greater than 5&nbsp;GHz, the losses (due mainly to the dielectric separating the inner and outer tubes being a non-ideal insulator) become too large, making [[Waveguide (electromagnetism)|waveguides]] a more efficient medium for transmitting energy. Coaxial cables often use a perforated dielectric layer to separate the inner and outer conductors in order to minimize the power dissipated by the dielectric.

==== 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.

==== Fiber optics ====
At frequencies greater than 200&nbsp;GHz, waveguide dimensions become impractically small, and the [[ohmic heating|ohmic losses]] in the waveguide walls become large. Instead, [[fibre optics|fiber optics]], which are a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.{{citation needed|date=October 2024}}