What is the significance of a negative imaginary component in a S21 measurement ?
For example, in the following plot of a passive network, the linear magnitude of the S21 parameter decreases with frequency monotonically. IOW: It acts as a low-pass filter.
However, the imaginary component is not monotonic - it has a minimum at 1.56MHz and is always negative (below the Ref 0U). What does that negative sign signify ?
If this were an Impedance plot (it is not), then the negative sign would signify capacitive reactance.