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You have be a little more specific. S11 and S22 are just input and output impedances. Magnitude plots of each would be return loss. This is how close the impedance is to the S parameter transmission line impedance. -20 db return loss is a close impedance match. It represents a small circle at radius = 0.1 around the center of a Smith chart.
they simply represents reflection back... e.g.
if S11=-10db
or 10Log(a/b)=-10db (where a is port 1 and b is port 2 of any circuit..( Twoport network) )
log(a/b)=-1
taking antilog....
a/b=0.1
or a=b/10
means if you insert a signal in port a.... 1/10th part will refelect back andremaing 9/10 part will be transferred forward.So they simply tell you behaviour of you circuit.
Kindly study : https://en.wikipedia.org/wiki/Scattering_parameters
if you want a simple and straightforward answer, that is if your S11 and S22 are below -10dB, you can probably send for fabrication. all the more better to tune them to less than -20dB. they basically tell you that you have good input and output matching to 50 ohm ports (i assume they are 50 ohm).
S11 is refelected input power and s22 is reflected output power.
In magnitude, we can say input return loss and output return loss.
Explanation for your query with example:
Assuem your passband is 500MHz to 600MHz, Impedance of your syatem is 50 ohm
Case 1: If return loss of S11 is 20dB in the band of 500MHz-600MHz, then max power (approx) transfer from input to ouput and vice versa.
Case 2: If return loss is less than 20dB in the band of 500MHz-600MHz, then less power transfer from input to output than case 1 ans so on..
Note: Outband of band no return loss available.
If you have still doubt, tell me your device (like amplifier, filter and etc..) and I will tell you more information.
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