Given how it's described as working, I'm pretty sure it's a descendant of the modern Active Denial System, which is good out to about 700 yards according to a quick Wikipedia check. Call it Sporting Rifle ranges and you'll be close enough.
As for power drop at range... since it's not actually trying to inflict damage, and microwaves are much better at pushing through smoke and such than photons, I think the dispersal of the energy would be trivial enough to just ignore, given how niche the thing is to begin with.
