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From: Stefan Jahn <stefan@gr...>  20070313 19:57:31

Am Di, 13.03.2007, 19:49, schrieb walter steffè: > Hello Stefan, Hi Walter, > I think that it should be nice to have implemented both ways. Ok, I see. > The waveguide line is usefull to connect different components which > interface > trough waveguide ports. > > The frequency variable in AC mode can be used to define a frequency > dependent > load such as the characteristic impedance of a waveguide. > > Regarding the computation of the wave guide line parameters there is not a > very big difference with respect to the TEM transmission line. > The difference is really small if you look at the S parameters. > As for the TEM line it holds S11=S22=0 and S12=S21=1. > The phases of S12 and of S21 are equal to beta_wg * length > and beta_wg is given by: > > beta_wg=sqrt(beta^2 kt^2) > > beta is 2PI/lambda in free space (or in the TEM line) Yep, so far for an ideal 50Ohm TEM line... > kt is the eigenvalue of the Hellmoltz equation associated with the > given > mode. It depends on the waveguide section, on the mode type (TE or TM) and > on > the mode order. For the rectangular waveguide there this number can > computed > with a simple formula but I think it would be better to make a generic > waveguide component where kt is a user defined parameter. > Then if you want you can specialize this generic component for the > rectangular > waveguide and in this case the kt of all modes will be computed > automatically. > The user still should be able to define the number of modes. > > I have not spoken about alpha because I have assumed a lossless waveguide. > At the moment I do not remember a formula for the computation of the > attenuation parameter. I think you can find tabulated data (which depend > on > the waveguide/mode/material/surface finishing...) on books like Matthaei > Young, Marcuvitz ... I think that the definition of this parameter can be > left to the user who should know the losses of its waveguide. Have you already looked at the QucsTranscalc tool? There is synthesis and analysis for rectangular waveguide available. It also tells if there occur beside the TEM mode any TE or TM modes. When creating a waveguide component we can probably easily make a good model for the TEM mode. When higher order TE or TM modes occur it gets perhaps difficult because each mode has different Zl and gamma. Michael: What do you know about this topic? (Me is not too familiar with waveguides. :( ) > The only problem I see is that the S parameters I have spoken about are > those > that are normalized versus the characteristic impedance. This can also be > written as: Zc_TE=beta/beta_wg, Zc_TM=beta_wg/beta. > The rules for the computation of the S matrix normalized versus Zc are > similar > to those explained in your paper but you must take care that Zc is a > function > of frequency and that it can be different on the different ports. I don't yet understand it fully... :( But the Sparameter simulation results are normalized to the impedance given in the Sparameter port (e.g. 50 Ohm). When Zl is different you get appropriate reflections, etc., also depending on the frequency. Hope this helps, Stefan. 