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In your plot of s11 over frequency, the value of s11 in dB for your frequency of interest gives the return loss of your devices, adding to it, the frequency with maximum dip in return loss shows its resonant frequency. In your plot of s21, place a marker at the peak its ur insertion loss, if you want theroretical way then look at the other post you replied to. on top to the formulae given, insertionloss also includes the dielectric losses and other losses if any..
First of all, what is the type of device under consideration ? (1/2/3 ... port ?, antenna/amplifier/mixer/filter... ?)
I will give the answer for a simple 2-port device.
Return Loss (RL) = -20*log10(|S11|)
Given S11, compute the magnitude i.e. |S11|, perform the log and scaling operations to convert to dB scale. This is your return loss plot vs. frequency. Depending on the specifications, you can compute bandwidths of operations ,where RL > 10 dB, RL > 20 dB and so on... (note the negative sign in the definition, it is always a positive number. I have even some academic papers make a mistake with the sign convetion)
Insertion Loss (IL) = -20*log10(|S21|)
Insertion loss refers to the loss of signal power between two points when the device is inserted. So it does not make sense to talk of insertion loss for 1-port devices.
Even in 2-port devices, amplifiers are supposed to produce gain, so technically there should be no insertion 'loss' for them.
It is more common to talk of IL for devices like filters and mixers, in which case, just use the information from the S21 curve together with the definition above.