Every resonance will have a dip in the S11 showing that power has either been radiated or absorbed in the antenna. The resonant frequency is a the bottom of the dip. On a Smith chart display each resonance will form a circle. Two or more resonances close together interact and make the behaviour more complex than this.

Dear Shravan,

If you have, for example, the s11 plot along the frequency, the lower the s11 is, that is where your antenna is matched. That is to say, in your graph, where s11 is minimum, there is probably the resonance.

(You could also see the real and imaginary part, where the imaginary part is 0, your resonance will probably be there too).

Best regards,

Miguel

Of course the dip in the S11 (depth depending on coupling) is ONE criterion...but its always important to know if you are over- or maybe under-critically coupled. This can be seen from the phase of S11 plot or easier from the circle in the smith chart

Under ideal matched conditions, the voltage standing wave ratio (VSWR) is one. However, practically it is taken as two because of the existence of both forward and backward waves between the source and the load. When we used to map VSWR of 2 into return loss (S11 parameter), the value of S11 comes near to -10 dB. Therefore, when S11 versus frequency plot goes below -10 dB, then it is believed that the antenna is resonating at those bands of frequency. The highest deep of the S11 curve below -10 dB is the resonating frequency of the antenna. But an antenna can work effectively below -8dB.

The minimum value of S-parameter below -10 dB is said to be the resonance frequency of that design.