The energy emitted by the source in [tex]dt[/tex] is [tex]Pdt[/tex]
where [tex]P=500\,kW[/tex]. At [tex]R=25\,km[/tex] from the source this energy will be uniformly distributed in a spherical shell of radii [tex]R[/tex] and [tex]R+dR[/tex]. The volume of this shell equals
[tex]dV=\pi R^2 c dt[/tex]
where [tex]c[/tex] is the speed of light. Now you have to know that the energy density of an electromagnetic wave can be written as
[tex]w=\frac{1}{2}\epsilon_0 E^2[/tex]
with [tex]E[/tex] the elecric field amplitude. So
[tex]P dt=w\cdot dV[/tex]
..... and you find E!
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