Z0 =sqrt(L/C) is a low loss/frequency approximation to the more general Z0=sqrt( (R(s)+sL)/(G(s)+sC) ), where s is the complex frequency. At low frequencies, R and G remain fairly unchanged hence the constant characteristic or the wave impedance in terms of microwave theory. As the frequency increases the change on the freq. dependent losses, R(s) and G(s) becomes more prominent. Indeed the frequency dependency of the relative permittivity causes change on the PUL capacitance but this is relatively small compared to the other two. In short, it is the high frequencies when Z0 becomes more frequency dependent. Hassan's proposal has been in practice in microwave design for over 60 years (both insertion and reflection losses should be considered, though). With increasing frequencies SI is converging to the same point. Is it a coincidence that prominent SI tools has included S-parameter analysis option in their feature set? Regards. Ihsan On 1/17/07, ryansatrom <ryan.satrom@xxxxxxxxxxx> wrote: > Hassan- > > It has always been my understanding that the instantaneous > characteristic impedance (at any point) of a transmission has only a > slight frequency dependency. > > As we all know, Zo =3D sqrt(L/C). > > Inductance has a frequency dependence only at low frequencies, where > there is an internal inductance caused by the fields internal to the > conductor. But as frequencies increase, skin effect causes all the > fields to travel on the exterior. At this point, the instantaneous > inductance of the transmission line is for the most part constant. > > As for capacitance, I've always thought that it was a function of the > size of the conductors and distance between the two conductors. Is > there any frequency dependency to that? > > Now, return loss is measured from input to output of any given > structure. Therefore we don't view return loss as "instantaneous". > So when I think of impedance and return loss I see two different > (although very related) values. > > Am I right? Is there something I'm missing? > > Regards, > Ryan Satrom > Everett Charles Technologies > > ------------------------------------------------------------------ > 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: > http://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: > http://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 > > > ------------------------------------------------------------------ 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: http://www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: http://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