Answer:
d = 2,236 m.
Explanation:
The received power on Earth, can be calculated as the product of the intensity (or power density) times the area that intercepts the power radiated.
As we assume that the transmitter antenna is ominidirectional, power is spreading out over a sphere with a radius equal to the distance to the source.
So, we can get the power density as follows:
I = P /A = P / 4*π*r², where P = 20 W, and r= 25 billion km = 25*10¹² m.
⇒ I = 20 W / 4*π* (25*10¹²)² m²
The received power, is just the product of this value times the area of the receiver antenna, which we assumed be a circle of diameter d:
Pr = I. Ar =( 20W / 4*π*(25*10¹²)² m²) * π * (d²/4) = 10⁻²⁰ W
Simplifying common terms, we can solve for d:
d= √(16*(25)²*10⁴/20) = 2,236 m.
Actually, the receiver antenna will have some gain (which it means that will concentrate more power in a given direction) so the real diameter will be less than calculated for an isotropic antenna.
Typical gain values for ESA (Earth Station Antennas) are in the range of 45-50 dBi.
