  This paper presents a detailed evaluation of the envelope-tracking adaptive
integral method (ET-AIM), an FFT-accelerated algorithm for analyzing
electromagnetic scattering. ET-AIM is used to solve progressively more complex
benchmark scattering problems and key parameters of the method (the auxiliary
grid size, near-zone size, temporal basis function type, time-step size, and
iterative solver tolerance) are optimized. The computational costs and accuracy
of ET-AIM are compared to its time-domain counterpart, the time-domain adaptive
integral method (TD-AIM), in the high-frequency regime, where the spatial
discretization of the scattering object is determined by the minimum wavelength
of interest rather than its geometrical features. Numerical results show that
although ET-AIM and TD-AIM computation times are comparable when the bandwidth
of interest is wide, the ET-AIM marching costs are dominated by iterative
solution rather than scattered-field computations ('right-hand-side'
computations) and that as the bandwidth of interest becomes narrower than 50%
of the center frequency, ET-AIM computational costs become significantly
smaller than TD-AIM ones. ET-AIM is also shown to efficiently solve large and
complex scattering problems whose solution by TD-AIM is impractical.
