It's a popular media cliche that aliens could theoretically have been listening to all human communication since radio was invented.

However the only type of radio communication that is emitted powerfully enough for aliens to listen to is analog color TV broadcasting which is not used anymore. For all of human history it was only during a few decades we actually broadcasted something aliens can listen to and then we stopped.

However what about radar? Military radars use frequency jumping but almost all civilian radar systems are blasting high power signals into space with more intensity than any of our communication signals.

Could SETI in theory listen to alien airport and weather radars?