If we gave a shit what an EMP here might do to the rest of the universe, then we could try to measure how the effect lessens as it travels, but the burst of microwaves traveling at the speed of light from 300 miles away won't be doing much fading in the microsecond of travel between the blast and the earth.

It does, and if we gave a shit what an EMP here might do to the rest of the universe, then we could try to measure how the effect lessens as it travels, but the burst of microwaves traveling at the speed of light from 300 miles away won't be doing much fading in the microsecond of travel between the blast and the earth.