I am getting puzzling inconsistencies between results for the reflection spectrum of a scatterer when I vary the width of the Gaussian pulse. Since I am a beginner this probably stems from my lack of understanding of something.
The scattering antenna is a metal loop with a small gap (i.e. the loop is not closed). The conductor cross-section is rectangular, 0.2 wide and 0.1 thick (all Meep units). The inner radius is 0.868, and the gap is 0.2. The antenna lies in the xy plane, centred at the origin. The region size is 4p x 4p x 6p, where p=6, and the PML thickness is p. The plane wave source is located at the lower edge of the PML, i.e. at z = -2p, and the flux plane is at z = -p, halfway between the wave source and the antenna. The frequency I am interested in is 5.5 GHz, i.e. a Meep frequency of 0.1833 with a Meep unit distance of 0.01m. This corresponds to a Meep wavelength of 5.456, which explains why I am using p=6. I am using resolution=20. The Gaussian pulse that is exciting the reflection has a centre frequency of 0.1833, and I have simulated various pulse widths, always with nfreq=100. With df=0.1 the reflection spectrum has a well-defined minimum at (0.139, 1.3E-06), and a flat local minimum at (0.181, 8.35E-6). With df=0.2 there is just a very flat minimum at (0.178, 1.97E-6), and with df=0.3 a minimum at (0.1, 1.61E-6). Zooming in with df=0.05 shows just the local minimum at (0.183, 8.4E-6), more like a point of inflection. Are these results surprising? Am I making an obvious mistake? I would be grateful for guidance. Thanks Gib
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