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Transmission Line Simulation

  • Nick

    I've been an avid user of QUCs now for about a year.  I use it mostly for coaxial transmission line problems (i.e. power dividers and low pass filters) as well as Filter analysis.  I have encountered something interesting though.

    Because QUCs only seems to do metric units, I often create a variable 'm' and set it equal to 0.0254.  This way I can still use english units, but just multiply by 'm' to convert to metric.  This works great.

    When I use that method for declaring transmission line sizes, i.e. 4" transmission line (IDOC = 3.995*m) which should convert the inside diameter to meters, and then call out the inner conductor size (ODIC = 1.711*m), I get an error "WARNING: Operating frequency (7.04e+008) beyond cutoff frequency (6.58539e+008)".  My sweep is from 596MHz to 704MHz.  I know for a fact that 704MHz is not above the cutoff for 4" coaxial transmission line. 

    As my current workaround, I simply resize everything.  Instead of using 3.995*m, I use 3.995e-3 to ensure I don't get this warning.  This obviously is not ideal, but it does seem to give agreeable results.

    • Stefan Jahn
      Stefan Jahn


      Right now I am not sure, but I think writing "mil" as unit should work.  Anyway, about the cut-off frequency:  I already saw something "wrong" in the calculation here.  On the one hand I think I missed the sqrt(er) factor for the TE(1,1) cutoff.  For the TM(n,1) cutoff I am not yet sure.  I'll investigate it.  Please remeind me if I forget!

      This is all just after having a quick look.  I need to do some tests...

      Anyway, thanks for spotting me to this!

      Hope these notes make sense, Stefan.

    • Stefan Jahn
      Stefan Jahn

      Hello Nick,

      sorry for the inconvenience.  I fixed the formulas for the cut-off frequencies for the coaxial line in documentation, the QucsTranscalc tool and in the simulator in current CVS.  The changes will be available in the next release then.  Anyway, the messages you got were only warnings...

      Regards, Stefan.