I took the one-sided FFT of a signal and plotted up until the Nyquist frequency. Then, I took the real part of this FFT multiplied by $i\omega$ following a calculation that I'm trying to do of a physics quantity related to the complex shear modulus (see link in the comments below; I don't think the physics is directly relevant, though).
This latter plot, $Re(i\omega FFT)$, follows my intuition for what this physical quantity should be, except for the large dip at high frequencies just below the Nyquist. About half the values are captured in this dip in log-log space. Since this is the FFT multiplied by $i\omega$ and then taking the real part, could doing this affect the value of the Nyquist frequency? Or could there be aliasing in certain cases below the Nyquist? I am trying to understand if this dip is physical or not.
Here is the FFT:
And here is $Re(i\omega FFT)$: