Understanding Dielectric Constant for Microwave PCB Materials

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  • Author: John Coonrod & Allen F. Horn III
One of the most important properties of a laminate used in microwave PCB technology is the dielectric constant, also known as relative permittivity or εr. Many electrical engineering courses teaching microwave technology will use this property as a constant for a given material when discussing a PCB related topic. Because of that experience, some engineers are under the impression that the dielectric constant (Dk) of a laminate is a fixed value for all microwave scenarios. In reality this is not the case. The Dk value found on datasheets for laminates is a number that was generated by a specific test method, which may or may not apply to a specific microwave application. ply to a specific microwave application. This article discusses several topics and begins with the basic theory as to why the Dk value of a PCB material can vary for different microwave scenarios. Some of the common test methods used for determining the Dk of microwave laminates will be explained as well. Lastly, analogies will be given between the various test methods and some general microwave applications.
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