Re: NonlinearFit problem

• To: mathgroup at smc.vnet.net
• Subject: [mg66553] Re: NonlinearFit problem
• From: "Scout" <Scout at nodomain.com>
• Date: Sat, 20 May 2006 04:46:46 -0400 (EDT)
• References: <e4ekoe\$9fa\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```"Oliver Friedrich" <frixoli at qarcor.de>
news:e4ekoe\$9fa\$1 at smc.vnet.net...
> Hallo,
>
> I want to use the NonlinearFit algorithm to obtain the parameters of a
> lossy line. There is a model
>
> Cell[BoxData[
>    RowBox[{
>      SqrtBox[
>        FractionBox[
>          RowBox[{
>            RowBox[{"2", " ", "\[ImaginaryI]", " ", "f", " ", "L", " ",
> "\[Pi]"}], "+",
>            RowBox[{
>              SqrtBox["f"], " ",
>              SqrtBox[
>                RowBox[{"2", " ", "\[Pi]"}]], " ",
>              SubscriptBox["R", "skin"]}]}],
>          RowBox[{"G", "+",
>            RowBox[{"2", " ", "\[ImaginaryI]", " ", "C", " ", "f", " ",
> "\[Pi]"}]}]]], " ",
>      RowBox[{"Tanh", "[",
>        RowBox[{"1.`", " ",
>          SqrtBox[
>            RowBox[{
>              RowBox[{"(",
>                RowBox[{"G", "+",
>                  RowBox[{"2", " ", "\[ImaginaryI]", " ", "C", " ", "f",
> " ", "\[Pi]"}]}], ")"}], " ",
>              RowBox[{"(",
>                RowBox[{
>                  RowBox[{"2", " ", "\[ImaginaryI]", " ", "f", " ", "L",
> " ", "\[Pi]"}], "+",
>                  RowBox[{
>                    SqrtBox["f"], " ",
>                    SqrtBox[
>                      RowBox[{"2", " ", "\[Pi]"}]], " ",
>                    SubscriptBox["R", "skin"]}]}], ")"}]}]]}], "]"}]}]],
> "Output",
>  CellLabel->"Out[166]="]
>
> that describes the input impedance of a lossy line when shorted at the
> end. The line parameters to obtain are Rskin,L,G and C. f is the
> independant variable. I have data of the line of the form {{f1,Z1},
> {f2,Z2},...} where Z is of course complex.
> Unfortunately NonlinearFit seems to have problems with complex numbers
> since it returns with messages saying "Objective function isn't real
> at...".
> Does anyone knows how to fix NonlinearFit for complex numbers or any
> other numerical method that could solve my problem?.
>
> Thank you
> Oliver
>

Why not with Amplitude and/or  Phase of the complex expression?

```

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