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I have some problems in understanding how to calculate the input (is it input?) impedance of a small circuit. (I have attached the equiv. circuit with values )

The circuit is a small loop antenna that resonates at 13.56 MHz and is tuned to 50 ohm.

From the source point of view: a capacitor in series (Cmatch = 11.74 pF) and then 3 elements in paralell; a capacitor (Cres = 96.76 pF), resistance (Rp = 20050 ohm) and an inductance (L=1.27 microhenry)

The impedance seen from the source is 50 ohm, however I don´t understand how to reach that answer,as calculating with complex impedances should be straight forward:

[tex]Z = \frac{1}{\frac{1}{Rp}+\frac{1}{j\omega L}+j\omega Cr} +\frac{1}{j\omega Cm}[/tex]

Solving this equation with matlab takes me faaaar away from 50 ohm ....

Am I doing something fundamentally wrong???

(The circuit and values are from a Texas Instruments Paper, should give impedance = 50 ohm.....???)

any help is appreciated

eirik

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# Impedance of load

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