RF Circuit Designer's Notes

Little nuggets of RF/analog circuit theory and design. Learn with me about PLLs, Q, noise, oscillators, filters, digital receiver concepts, etc.

Wednesday, November 01, 2006

How to calculate S21, S11

S21 calculation

Connect a 2V, 50Ω source to the input of the two port network.
Connect 50Ω load to the output.
Calculate the voltage at the output.
This voltage is S21.

In dB, S21 magnitude = 20*log (Vout)

S11 calculation

Terminate output with 50Ω.
Calculate input impedance and reflection coefficient.

r = (Zin - 50Ω)/(Zin + 50Ω)

S11 (dB) = 20*log(r)


Single element two port networks

For a network with a single element (Z) in series between the input and output.
S21=2/(2+Z/Zo)
S11=Z/(2*Zo+Z)

For a network with a single shunt element (Y)
S21=2/(2+Y*Zo)
S11=-Y*Zo/(2+Y*Zo)

3 Comments:

At 1:13 AM, Anonymous Anonymous said...

in order to measure s21 and s11 by using VNA and the antenna apparatus, does the distance between the antennas will affect the results?

 
At 12:30 AM, Anonymous Anonymous said...

to the magnitude, it won't change. but as the length of transmite line changes, the phase of RF signal will change accordingly. so if you watch the wave in the smith chart, you will get a different waveform

 
At 12:47 PM, Anonymous Anonymous said...

Great page! thanks for the info...

 

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