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

Fri Jan 12 11:16:58 EST 2018

```On 1/12/18 7:57 AM, Steve Hunt wrote:
> Yes!!!! - S11 Magnitude and Phase are quite different from S21 Magnitude
> and Phase.
>
> If you are interested, here's how I derive the choke's complex impedance
> from S21 Magnitude and Phase:
> http://www.karinya.net/g3txq/chokes/s21.pdf
>

The conversion of Mag/Phase to complex values is the same, but the
conversion of the measurement to Z is different...

in the 1 port measurement, the unknown is in series with the 50 ohm
impedance of the source, so you're looking at the ratio of the
voltage/current across the Z compared to the voltage/current across the
series combination (with some math to turn it into gamma/S11)

In the 2 port measurement, the unknown is between the ports, so you're
looking at the voltage/current across the port 2 load, compared with the
voltage/current at the port 1 source (with some math to turn it into S21)

Steve's website has the equations.

```