# [TowerTalk] S21 to Rp & Cp - Equivalent Circuit

Sun Jan 7 17:01:54 EST 2018

```On 1/7/18 12:59 PM, Máximo EA1DDO_HK1H wrote:
> Hi all,
>
>
> I know there is a formula to get Rp and Cp from S21 or S11 data (LogMag and Phase, or R & I).
Convert S11 from logmag, angle to complex

Z = Z0 * ((1+gamma)/(1-gamma))
in Excel
gamma =
imcomplex(10^(S11mag/20)*cos(s11phase*pi()/180),10^(S11mag/20)*sin(s11phase*pi()/180))

Z= immult(50,imdivide(imsum(1,gamma),imsub(1,gamma))

that gives you R+jX series form R and C/L

You then have to convert to the parallel form

Rp = Xs*(Rs/Xs+Xs/Rs)
Xp = Rs*(Rs/Xs+Xs/Rs)

The Rs/Xs+Xs/Rs is common to both, so in excel, adding a column for that
makes it easier.

>
> Anyone know that formula?
>
> It is to use it in Excel and get Rp and Cp from the VNA´s exported data.
>
>
> Thank you very much
>
>
> 73, Maximo
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```