Given intesnsity of a radio signal and the distance, find the power of the source.

1. Nov 6, 2009

jabreedlove

1. The problem statement, all variables and given/known data
Regulations require that licensed radio stations have limits on their broadcast power so as to avoid interference with signals from distant stations. You are in charge of checking compliance with the law. At a distance of 25 km from a radio station that broadcasts from a single vertical electric dipole antenna at a frequency of 1140 kHz, the intensity of the electromagnetic wave is 2 multiplied by 10-13 W/m2. What is the total power radiated by the station?

2. Relevant equations
I think we should use I = P/A, rearranged to P = AI where P is the power of the source, A is the area, in this case the surface area of a sphere, 4 pi r^2, and I is the intensity

3. The attempt at a solution
Using the equation above, we came up with an answer of 1.571 mW, and the website says the answer should be 1.05 mW. What seems suspicious to me is that the frequency should be used, and we couldn't find a way to use it.