# Convert S11 to Z11

Question asked by snowbob78 on Oct 6, 2011
Hi, I am trying to verify some S11 measurements I took, here is an example at 2 MHz:

Frequency          Inductance (L)          Z11              S11
2.00E+06             -0.000659        141.682350          0.999721

Inside there are these variables:
Frequency : the frequency span
S11 : the return loss (S11)
Z11 : the complex impedance
L : the Inductance

The Network Analyzer gives you only the S11 data.
To convert S11 to Z11 we use the formula
Z11 = 50*(1+S11)/(1-S11)
The impedance has a real and imaginary part. The real part corresponds to
the "resistance" of the coil, which should be very low (and it is low).
The imaginary part (reactance) can be considered either an inductance or a
capacitance, depending on its sign. We assume that the reactance is always
positive. In this case
L = (i*imag(Z11))/(i*omega)

I used the Z11 formula above and plugged in the measured S11 parameters but did not get the Z11 in my example above.  What am I doing wrong here?