An explanation:
The observing frequency, 1420 MHz, corresponds to the wavelength of radiation produced by a quantum mechanical "spin-flip transition" in neutral hydrogen atoms. Because hydrogen is the most abundant element in the universe and because 1420 MHz is a relatively quiet frequency free from sources of natural radio interference, many SETI experts think that 1420 MHz is a good place for interstellar broadcasts. Would aliens reason the same way? There's no way to know, but the search has to start somewhere.
http://science.nasa.gov/science-news/science-at-nasa/1999/ast23may99_1/
But really this isn't about astronomy. It is more my own signal for my family and friends; a way to tether home.
