> I'm trying to generate a LFM Chirp source i.e. freq = f0 +K*t, where K > is the chirp rate. When I do the envelope of the curve always seems to > grow bigger. I've tested my chirp function itself and it appears to > work > fine. I even put in a slow rising envelope (similar to the built in > sinusoidal function) and that didn't help.
What you are seeing, I suspect, is a nice physical demonstration of the density of states. As the frequency of the current grows, the amplitude of the field will grow. Just try putting in a continuous- wave source and varying the frequency, and you'll see that increasing the frequency will increase the amplitude of the resulting field. The reason for this is related to the reason that the sky is blue (scattered power increases with frequency). This is sometimes phrased in terms of the density of states, which increases as the frequency to the d-1 power in d dimensions. This is a common confusion with Meep users. The current is not the same thing as the resulting field, and the current amplitude alone doesn't determine the resulting field amplitude. Not only is it affected by the frequency, but it is also affected by any surrounding geometry (which changes the local density of states). A classic example is an antenna in front of a ground plane, which radiates very different amounts of power depending on the distance between the antenna and the plate (half wavelength vs. quarter wavelength, for example). A more recent example is a current source inside a medium with a complete photonic bandgap -- for frequencies inside the gap, no power at all will be radiated. Steven _______________________________________________ meep-discuss mailing list meep-discuss@ab-initio.mit.edu http://ab-initio.mit.edu/cgi-bin/mailman/listinfo/meep-discuss
