Editing Relative permittivity (section)

=== Communication ===
Dielectrics are used in [[radio frequency]] (RF) transmission lines. In a [[coaxial]] cable, [[polyethylene]] can be used between the center conductor and outside shield. It can also be placed inside waveguides to form [[dielectric resonator filter|filters]]. [[Optical fibers]] are examples of ''dielectric [[waveguide]]s''. They consist of dielectric materials that are purposely doped with impurities so as to control the precise value of ''ε''<sub>r</sub> within the cross-section. This controls the [[refractive index]] of the material and therefore also the optical modes of transmission. However, in these cases it is technically the relative permittivity that matters, as they are not operated in the electrostatic limit.