The Dielectric Constant is a convenient way of discussing the permittivity of materials. Recall that the permittivity of a vacuum (that is, in outer space or where there is no atoms or material in a volume - also known as Free Space) is 8.854*10^-12 [Farads/meter]. Suppose the permittivity of a material such as FR4 is to be determined. FR4 is a common dielectric used in circuit boards as the insulator between the ground plane and the signal traces. We measure the permittivity of this material to be 3.54*10^-11 [F/m]. Well, that's not particular easy to remember. In addition, we really only care about the ratio of the permittivity of the material to the permittivity of Free Space. This tells us, amongst other things, the slow-down of the speed of light within the material, as well as the corresponding decrease in wavelength for an electromagnetic wave within a material.
The ratio of the permittivity of a material (let's say given by ) to the permittivity of free space (written ) is the dielectric constant. That is:
Therefore, the material FR4 can be found to have a dielectric constant of 4. Almost makes you wonder what the 4 in FR4 stood for. In this manner, it is much simpler to discuss dielectrics by their dielectric constants, instead of their permittivities. Note that the permittivity of Free Space (vacuum) is written because it is the lowest permittivity found in nature. Every other material (including air) will have (at least somewhat) a higher dielectric constant.
I will conclude this page with a table of common dielectric constants.
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