Radio is daft.
Picking up radio is limited to a few light years range due to cosmic noise and inverse square law unless someone has massive dishes pointing straight at us. Even then the range isn't great. It's doubtful our own ordinary transmissions can even be received at the nearest star.
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three basic questions:
1 How abundant and diverse is intelligent life in the universe?
2 How does intelligent life communicate?
3 How can we detect intelligent life?
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1) We can't answer the first question at all.
2) No communications method we know is practical for interstellar distances, so forget this question, it's irrelevant. We aren't going to "eavesdrop". The inverse square law and Shannon's law mean that's probably impossible.
3) The only current practical method for Interstellar distances is spectroscopy. You are using the power of a star as the remote transmitter and detecting the adsorption in the atmosphere of a planet transiting between us and the star. Not all planets will transit between us and stars.
Unless we discover a whole new branch of physics we are limited to spectroscopy. We need to have no preconceived ideas and examine each spectrograph. Oxygen and carbon Dioxide has a signature. If there are additional chemicals that seem to be in unnatural proportions, we might conclude we have found possible evidence.
How do we communicate if we thing we have found life? Radio or Laser won't work, too far even if they were "looking" in our direction. We can send a probe that takes hundreds or thousands of years to get there.
We can't know if we discover unknown stuff in the future that allows "ansibles", stargates, warp drive, artificial wormholes or "jump" drive. Those are all SF today. Contrary to popular belief, Quantum Entanglement can't be used for instant communications but only to verify a light speed message hasn't been tampered with.
So Spectroscopy.
Biting the hand that feeds IT
