Water hole

The water hole is the band from roughly 1420 MHz to 1662 MHz. At the bottom is the hydrogen line, from neutral H. At the top are the hydroxyl radical’s lines, the OH that you get by splitting a water molecule. H plus OH is water, hence the name. Bernard Oliver proposed the idea in the early 1970s during the Project Cyclops study: if anyone else is doing radio astronomy, they know these lines, and the stretch between them is an obvious place to put a signal meant to be found.

The chemistry pun is the memorable part, but the stronger argument is physical. The radio sky has a quiet window. Below about 1 GHz, synchrotron emission from the Galaxy rises steeply and dominates the sky noise. Above about 10 GHz, the Earth’s atmosphere (oxygen and water vapour) adds its own. The cosmic microwave background sets a floor of about 2.7 K everywhere. The water hole sits near the bottom of that window, where a receiver on the ground sees the least natural noise per hertz. Lower sky noise means a lower system temperature, which means a weaker signal can be detected in the same time.

Is it actually quiet?

Naturally, yes. Artificially, only partly. The 1400 to 1427 MHz slice is protected for passive use, but the rest of the water hole is shared: satellite navigation, satellite phone downlinks and aeronautical services all live in or near it. Our working list of what turns up and how to rule it out is in the RFI entry. In practice a SAW-filtered LNA centred on 1420 MHz, as described in LAB-006, is what makes the band usable for a small station.

Why we’re here

This is the band Sixmile Run listens in and the band where we found the signal we publish translations of. We picked it for the same reasons everyone else does, and we are aware of how it reads when a garage station says it heard something there. LAB-001 explains the choice; LAB-005 covers the frequency itself. None of the math on this site depends on believing us: the antenna and noise calculations are the same for anyone pointing a dish at 1420 MHz.