Earth SETI is basically just looking for directed signals: hypothesised to be either intentional communication attempts directed at us, or signals that we happen to be in line-of-sight of. Omniidirectional signals that could be detectable over interstellar distances would require immense amounts of power at the transmission end, and anyway may even be detected by conventional radio astronomy observations.
The popular conception is that our undirected, historical tv/radio signals might be detected by ETIs - and that therefore this is what our SETI efforts are looking for. In practice, the former is unlikely using known scientific principles, and the latter isn't really being attempted for the same reason.
The popular conception is that our undirected, historical tv/radio signals might be detected by ETIs - and that therefore this is what our SETI efforts are looking for. In practice, the former is unlikely using known scientific principles, and the latter isn't really being attempted for the same reason.