The numbers given at the website you referenced are as follows for the Aricebo antenna:

Minimum detectable signal power = 10^(-27) Watts per square meter

A 1-watt signal sent in all directions from a transmitter 1 meter square in area will be 6.98^(-34) times the strength at a distance of four lightyears, at the nearest star systen, Alpha Centauri. That's 14 million times too weak to be detected by Aricebo. To be detected the signal would have to be over 14 megawatts per square meter antenna area.

The example they give is the United States Naval Space Surveillance System, which has an effective isotropic radiated power of 14 billion watts, they say detectable by an Aricebo-type dish at a distance of 60 lightyears. I don't really believe their equation is correct because it doesn't take into account the transmitter area, only the ratio of radiated-to-detected power. What is their standard standoff distance for effective isotropic radiated power? Put another way, what is their effective isotropic transmitter power-to-area ratio?