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On 07/13/2017 06:43 PM, Steve wrote:
My model of current interest is a small resonant cavity excited by a magnetron at ~2.45 GHz with a noise bandwidth of about 30 MHz feeding a coil antenna. The antenna is modeled as point dipole, Hz excitation and I make my Meep runs at 500 resolution. As one can see from the trend in the above it is not practical to guess at the change-source time which will result in a low value of the scaler fields. Unfortunately, a static field existing in the cavity has an effect on the behavior of the fields created by the new antenna. What you are observing is the net non-zero dipole moment left behind by the abrupt cutoff of the source. These are high-frequency components which are not absorbed by the PML. Note that sources in Meep are actually based on the integral of the time function. This is done to ensure that they have zero integral when smoothly turned on and off. For more information, see this archive post. Besides using "change-sources!", another way to investigate the effect of abruptly turning off the source is to use the "width" property of "continuous-src", as demonstrated in this simulation script for a 3d computational cell: (set-param! resolution 20) (set! geometry-lattice (make lattice (size 3 3 3))) (set! pml-layers (list (make pml (thickness 1)))) (set! symmetries (list (make mirror-sym (direction X)) (make mirror-sym (direction Y)))) (define-param wt 0) (set! sources (list (make source (src (make continuous-src (frequency 1.0) (start-time 0) (end-time 40) (width wt))) (component Ez) (center 0 0 0)))) (define print-field (lambda () (print "field:, " (meep-round-time) ", " (get-field-point Ez (vector3 0 0 0)) "\n"))) (run-until 1000 (at-every 2.3 print-field)) The "continuous-src" has an "end-time" of 40, identical to the previous example involving "change-sources!", with the only modification being the "width" property which, as described in the Meep reference page, controls how smoothly the source turns on and off. We run three different tests with the "width" parameter set to 0, 1, and 10 and plot the field intensity as a function of time. Larger values for the "width" cause the source to turn off more smoothly which therefore reduces the high-frequency net dipole left behind. More information is provided in this archive post, and another. Results are similar in 2d (plot). |
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