> Is there a principle behind it? In a nutshell, the property we call "dielectric constant" (permittivity or epsilon) is a complex quantity. The real part is associated with energy storage, the imaginary part causes energy loss or dissipation. Loss tangent is one way to measure this. An ideal good dielectric has no imaginary part. Permeability is also a complex quantity and has a loss component too, but loss tangent generally refers to permittivity's non-idealness. Most materials we use to carry electromagnetic fields have close to an ideal permeability. Andy ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List FAQ wiki page is located at: http://si-list.org/wiki/wiki.pl?Si-List_FAQ List technical documents are available at: http://www.si-list.org List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu