Measure Bandwidth of a LNA?

What is the BW of the LNA anyway? Taking a shot at the definition, is it just the range of spectrum where the LNA can amplify a input signal?

How would you measure the BW of a LNA with a VNA? Taking a shot at the definition, is it just S21 log mag across the entire frequency range of the VNA (30MHz to 20GHz)?

Please advise.

The bandwidth is generally defined as the range of frequencies over which a device performs to it's rated specifications. For example, if you have an LNA that is rated to have 20 dB of gain and a 1.3 dB noise figure, then the bandwidth will be the range of frequencies where both of those requirements are met (along with other specs that might be of interest, like IP3).

A VNA would be a good tool to use to capture the gain and return loss aspects of the LNA, but it may not give you any information about the noise performance (some VNA's have add-on modules and software that will enable them to analyze NF, but they are more expensive). For noise figure measurements, you should be able to find a Noise Figure meter and it's accompanying noise source.

As for measurement, yes, S21 on a dB scale (the LOGarithm of the MAGnitude). You could also look at S11 (input return loss) in dB scale and compare it to the spec sheet for return loss (it may or may not be listed).