This app can be used to find the power density in the main beam of the antenna. The user is prompted to enter the gain of a tower-mounted antenna in dB the transmitter power in Watts and the distance from the antenna in meters.

As an example enter gain = 40.0 P = 5.0 and R = 20.0 then push "Calculate' and your answer should be 9.95 W/m^2.