If we want to contact any of those other civilizations out there, we’ll need a beacon to send messages with. But what to build?
Gregory Benford at the University of California Irvine and family (?) have done a cost/benefit analysis on the types of microwave generators out there that can produce the 10^17 W necessary to reach a significant proportion of the galactic habitable zone.
There are various ways that the cost can be optimised and the Benfords summarise them like this:
“Thrifty beacon systems would be large and costly, have narrow searchlight beams and short dwell times when the Beacon would be seen by an alien observer at target areas in the sky. They may revisit an area infrequently and will likely transmit at high microwave frequencies, ~10 GHz. The natural corridor to broadcast is along the galactic spiral’s radius or along the spiral galactic arm we are in.”
This has implications for the search for ET civilisations (as opposed sending messages for them). If ET civilisations are as cost conscious as we are, then they may well have built beacons in this way.
And if so, say the Benfords, nearly all SETI searches to date would have missed them.
arxiv.org/abs/0810.3964: Cost Optimized Interstellar Beacons: METI
arxiv.org/abs/0810.3966: Cost Optimized Interstellar Beacons: SETI