Intensity is measured in Watts/meter^2
power is measured in watts
for a point source radiating some power P, the appropriate "meter^2" is the
surface area of a sphere of radius R, or 4 pi R^2
thus the intensity drops off as 1/R^2
most sources can be approximated as a point source
one exception is the antenna, which has a tendency toward directionality
you can make a first order approximation assuming a point source
beyond that, you'll need to have an approximation of the radiating pattern of your antenna