This is always so humbling but it's so tough to get a hold on. The thing people don't really get is just how weak those signals are by now. Radio waves decay by the inverse square law - intensity is proportional to one over the distance squared. So let's take a skookum transmitter. Let's assume it's been crankin' since 1910, instead of 1974. That gives us 1400 kW (1w/1m). Sounds like a job for Super Wolframalpha. Badass. 1.42E-12 W/m. Whenever you see E-12 you know you're dealing with itty bitty numbers. Out of curiosity, what kind of signal can, say, the VLA grab? We'll ignore, for a moment, the fact that it's three orders of magnitude outside the resolution. Well, shit, it's in flux density. Looks like it goes up as the frequency increases, too - so three orders of magnitude down, like we are, we aren't in good shape. Let's try and get this a little more real - how 'bout Mr. Drake Equation himself? Turns out Ole Frank was cruising around the 400kHz band when he did the first SETI snooping. What's the Green Bank Observatory look like? Well, shit. Apparently it's not that simple. You can get to their calculator if you're a credentialed scientist and have a login. I may have to call a credentialed scientist. But hey - in the meantime, that mammer-zammer tower has been broadcasting for 40 years. Here's what the neighborhood looks like for 50. Not so humbling anymore, is it?
I know it's a cliche by now but this was always a favourite envisioning of the concept. (For a film released in 1997.)