# How to determine the central frequency from a vectorial spectrum analyzer?

I have taken measures of an antenna with a vectorial spectrum analyzer. I need to obtain the central frequency at which the antenna works. In order to do so, I have selected S11 (reflection) and I have plotted the "log magnitude" of the spectrum, I obtained something like this:

I know that the central frequency is obtained from one of the peaks shown on the plot. But in this case, I obtain three peaks. Which one should I take if they are pretty much the same? (no peak is considerably greater than the others)

## 1 Answer

The S11 represents the relative amount of signal that is reflected from the antenna, so the lower the better, because lower means more radiated by the antenna.

Usually, we consider the antenna to be "working" at a given frequency if its S11 is lower than -10 dB (of course, this requirement can be changed according to the application. spatial antennas are more around -20 dB for example).

Therefore, your antenna seems to be working at three distinct frequencies (with the s11 < -10 dB criterion). Is this a problem ? It depends on the hardware you put behind it during operation.

• Thank you so much for your clear explanation. I was still confused about what's the S11 parameter, now I got it much clearer. Sorry because I cannot upvote your answer yet. – user3780731 Mar 12 '16 at 14:33