At the present time on Earth, some of the most important acquisition signals originate from a half-dozen or so U.S. military radars (and their presumed Soviet counterparts). These Ballistic Missile Early Warning System (BMEWS) radars sweep out a large fraction of the local horizon with extraordinarily powerful transmitters. The result is that this "radio service" provides by far the most intense signals that leak from our planet to a large fraction of the sky.
While BMEWS radars pass criterion (2) above, they fail (3) and partially fail (1) because they are so few and often change their frequency of operation to avoid being jammed. Nevertheless, if an external observer used equipment comparable to the most sensitive radio telescope on Earth (the 305-m diameter dish at Arecibo, Puerto Rico), we calculate that a BMEWS-type radar could be detected as far away as 15 light years. This distance includes only about 40 stars, but, of course, it is possible that our eavesdropper possesses a much more sensitive radio telescope than we do. If "he" had something like the largest one ever proposed for Earth, namely, the array of a thousand 100-m dishes called for by Project Cyclops (Oliver and Billingham, 1973), he could detect a BMEWS-type radar at a distance of 250 light years. In this case at least 100,000 stars are possible candidates for such an eavesdropper's location. But note that radio waves travel at the finite speed of 1 light year per year, and thus it will take until the 23rd century, or 250 years from now, before all these stars have had a chance to be bathed in the radiation of our defense system radars!
After picking up a BMEWS (or other) acquisition signal, the observer needs at least 100 times more sensitivity in his equipment to reach the rich lode of information signals emanating from Earth. It turns out that television broadcast antennas (or stations) are the most intense sources of such signals.