How to see if microstrip line is radiating or not?

margarida said:

I have found a 4.5dBi total gain

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I am quite sure that this is directivity, not gain.

Reference for gain is the power into by the device. This should be really small for a transmission line.
Reference for directivity is the total radiated power, so that you can see large numbers even if the amount of radiaton is very small.

Gain = k*Directivity, where k (dimensionless) is the efficiency factor (0 ≤ k ≤ 1)
Due to symmetry unbalance in microstrip line, all discontinuity elements make the line to radiate somehow, nevertheless its dimensions.
When the microstrip line was invented in 1952, radiation was the main issue of this RF component.
If compare a microstrip line to a patch antenna you will not see much differences, other than dimensions.

vfone said:

all discontinuity elements make the line to radiate somehow

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That is true, and we can calculate the maximum total loss (dissipation plus radiation) from the S-parameters given in the initial post.
S11 -25dB means S11=0.0562
S21 -0.2dB means S21=0.9772

Total loss (dissipation + radiation) is then 1 - |S11|^2 -|S21|^2 = 0.0419
So for this model, sum of radiated and dissipated power is 4% of the input power. This already shows that the model has only little radiation.

To get the radiated percentage (excluding dissipation loss) we can simulate a lossless model and do the math above. Or as you mentioned, we can look at the "radiation efficiency" parameter.

volker@muehlhaus said:

That is true, and we can calculate the maximum total loss (dissipation plus radiation) from the S-parameters given in the initial post.
S11 -25dB means S11=0.0562
S21 -0.2dB means S21=0.9772

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Why do you use 20*log10 instead of 10*log10?

margarida said:

Why do you use 20*log10 instead of 10*log10?

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S-parameters are voltage, not power