Effective Dielectric Constant
CHAPTER 44.8 Effective Dielectric Constant
If the material between the signal trace and the return path is “completely” filled by a dielectric material, all the electric field is contained within the dielectric material. The examples of such structures are stripline and the coaxial cable.
Some structures contain only part of the electric field within the dielectric material. Examples of such structures are microstrip, twisted pair cable and ribbon cable.
In case of the stripline the capacitance per unit length increases by a factor which depends upon the dielectric constant of the material between the trace and the return plane. If there would have been air between the trace and the return plane, this factor would have been unity.
In case of the microstrip, part of the electric field is in air and a part is in the dielectric. If the dielectric constant of the material is er, the effective dielectric constant of the structure for the calculation of capacitance per unit length is somewhere between 1 and er.
If we increase the width of the microstrip, more and more electric lines of forces are within the dielectric field. The effective dielectric constant therefore increases with the increase in the trace width.
The following formula can be used to calculate the effective dielectric constant.
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