Avner-- If I am wrong, I hope someone out there will make me smarter, but the dielectric "CONSTANT" is just that--constant for that type of material. IMOP, when calculating impedance of microstrip or stripline, the dielectric constant of a material doesn't change, less the material's unique tolerance range REF: IPC-D-275 Table 3-2. Therefore, unless proven wrong, the constant DOES NOT change, but the impedance value WILL vary depending on which formula you are using depending on you pwb construction. Hope that helps. If wrong, please correct me via E-mail, Bob Vanech E-Mail : [log in to unmask]