I get reliable detection out to about 25-30 km, but it depends on your placement and local RF environment. I hung my Acurite sensor on a nail up under a roof eave out of direct sunlight so as to get reasonably accurate air temperature readings and that seems to be pretty good for lightning as well. I'm in a bit of a valley which probably limits my range somewhat, I am fairly confident the 40 km range is possible with the right placement.