[Johnny555]

If the aliens want us to know they are there, it won't be because we have a specialized radio dish to receive their transmissions -- it's unlikely that we'd be able to distinguish their regular communications from noise. In another 50-100 years, the earth will likely stop transmitting the kind of high power, trivially modulated signals that we're looking for, which leaves a very small window of opportunity to detect other alien species.

It'd be like Marconi trying to decipher a modern spread-spectrum encrypted RF datastream.

[Animats]

I pointed this out to some SETI people over a decade ago. They were focused on looking for "carriers", signals with a strong center frequency component. Analog TV, was AM video with 80% carrier, requiring huge transmitter power. UHF stations used to emit megawatts. Those signals are gone. More modern transmissions (DTV, cellular) look like white noise across a narrow chunk of spectrum. Those are much harder to pull out of the noise background. You can detect that a carrier is present even if you're missing most of the signal. That's not true of more modern modes.

If a civilization wanted to be detected, they could easily send out clearly understandable signals - big carriers with slow modulation. Arecibo could in theory communicate with a similarly sized dish across the galaxy. An answer would be a long time coming, though.