when we calculate S parameter in DB

DB is a ratio what ratio takes plase in the S parameters measure?

thanks

S parameters is ratio between powers of incident and reflected waves. As exemple for two port device: S21=20 dB means, that power of incident wave a1 is 100 times smaller than power of reflected wave b2.

An one more thing

S21=b2/a1, this is a complex value. When we talk about gain (measure it):

Gain=|S21| and Gain in dB=20log10(|S21|)

thanks alot

one note.
dB generally has no physical quantity, which makes it convenient to use between people interested in amplitudes and powers. So, the basic rule is dB=10log10(P/Po)=20log10(A/Ao). Obviously, 2 comes from square of amplitude. Since S-params are about amplitude, it is "20". But in measurements, when treating detected envelopes etc. you often take it with "10xlog" from the screen data.
dBm, dBc, dBa etc, are just relative cases with different Po for different measurements.