Calculating, or even defining, power density in the near field is a bit
dicey to say the least. But the field strength (E or H, which aren't
necessarily in phase or oriented at right angles in this vicinity) can
readily be determined. Here are some values for the E field as a
function of distance from the center of an octagonal loop about 3 feet
in diameter at 7 MHz, with 100 watts of applied power, compared to the E
field calculated using the proposed simple spherical method (and further
assuming, incorrectly, that the wave impedance is 377 ohms resistive):

Dist m E V/m E sph apx V/m
1 683 54.8
2 133 27.4
4 34.2 13.7
8 11.1 6.85
16 4.58 3.42
32 2.15 1.71
64 1.06 0.856
1000 0.0548 0.0673

As you can see, the approximation might be adequate at some distances
and for some purposes but not for others.

Roy Lewallen, W7EL