matt schrieb: > > > > Even with your settings, it is possible to discern the existence of a > scattered > field, when no scatterer is present. > > I think the problem isn't numerical dispersion per se, but rather the > discrete > nature of your sources. I think if you use line sources instead of > individual, > discrete point sources, this should disappear.
Actually in the end you have a discrete grid, so even a line source is discretized to individual point sources. But of course the correct position of the source also plays a role. As described on the meep homepage, meep distributes the sources to the adjacent grid points, but in my code I specify the electric and magnetic current at the same position. Because of the shifted grids for H and E field at least one of this currents is not where it is supposed to be, which could limit the accuracy of the planewave. > > One thing I didn't understand were the cy and cx parameters. Also, in the > first part of your original code you had 4 pixel shift which doesn't appear > in > the second part. Was there something different? cx and cy are the phase velocity along the axes. Do you mean this shift: (define numPix 4);how much cells the monitors should be shifted ? This has nothing to do with the source itself. It is in the file with the broadband source and specifys how many pixels the flux monitors are shifted inside the TF area (absorbtion monitor) and outside the TF area (scattering monitor). I used this to calculate absorption and scattering cross sections of some scatterers. I place a close surface inside the TF area where the recorded flux tells you how much Energy flows into the scatterer and one closed surface in the SF area where the scattered energy is recorded. > > Best, > Matt > > > > > On Wed, 30 Jan 2008, Andreas Unger wrote: > >> Hi matt, >> >> This seems to be a problem of numerical dispersion introduced by the >> finite grid. When I change the parameters to >> >> (define-param freq 5);frequency >> (define-param fcut 5);cutoff >> (define-param fw 0.1);frequency width at turning on the source >> >> (define Lpml 0.5) ;thickness of the pml >> (define L 2 ) ;total size of the space >> (define tfSize 0.9);edge length of total field area >> >> and >> >> (set! resolution 250);Set the resolution >> >> then the planewave looks nice again. Actually this TFSF works only fine >> for relativly small TF areas. The reason is that it requires very >> precise timing for turning on the sources and annihilate the outgoing >> waves. In the Amplitude functions along the boundary the analytic >> wavevectors for the given frequency are used but in reality the >> wavevectors are a bit different due to numerical dispersion. The bigger >> the TF area gets compared to your wavlength the stronger you will see >> this! Mybe this effect can be compensated by using slightly different >> wavevectors but I have not tried. >> >> Greetings, >> Andreas >> _______________________________________________ meep-discuss mailing list [email protected] http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss
