posted a question
Consider a civilization broadcasting a signal with a power of 1.8�104 watts. The Arecibo radio telescope, which is about 300 meters in diameter, could detect this signal if it is coming from as far away as 134 light-years. Suppose instead that the signal is being broadcast from the other side of the Milky Way Galaxy, about 70000 light-years away.

How large a radio telescope would we need to detect this signal? (Hint: Use the inverse square law for light.)

Diameter in Meters.