K_effective

Updated August 18, 2020

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New for June 2010! Here we will explain the term "Keff" which is short for "K effective".

"Dk" is often used as the "dielectric constant". Why not "DC" you ask? Unless you are not an electrical engineer, you'd realize that DC was already taken as "direct current!"

In this context, "k" refers to the effective "dielectric constant", which might also be called Epsilon effective.

The dielectric constant Dk, also known as epsilon relative (ER) is a material property, a measure of the material's property of slowing electromagnetic waves down. The parameter ER is usually nearly constant with frequency, which is why it is called the dielectric constant. Sometimes ER is anisotropic.

Given that a two-conductor transmission line is transverse electromagnetic (TEM) and is fully encased in a uniform material, the speed would be c/SQRT(ER). Dk is also the parameter used in capacitance calculations, Dk is proportional to capacitance value for a given area and plate separation.

Epsilon effective (Ee), a.k.a. Keffective, takes into account geometry of a transmission line, which makes it less than ER (or Dk) in many cases. For microstrip, part of the wave is in air, and part is in the substrate material. Once a "filling factor" is calculated for what percentage is in the material, then the Epsilon effective is equal to ERxFF+1x(1-FF). The second term comes from the percentage that is in air (ER=1). Most calculations for microstrip ignore filling factor and directly provide Keff and Z0.

Here's a page that explains filling factor.

Here's a page on how to extract Dk from transmission line measurements.