[MrB (Simon)]

Originally Posted by Astro78

http://www.newscientist.com/article/...se.html?page=1

Can anyone please lend some light on these radio frequencies that are banned under international agreement for #11, please?

From everything2.com:
1420 MHz is the frequency of precession of the single orbital electron of the hydrogen atom. Thus, it is a frequency that is constant wherever there is hydrogen (i.e. everywhere). This frequency is being used by SETI, because it is thought that all fairly advanced civilisations will know of this magical frequency. No matter what units of measurement they use, it will mean something to them.

From exploritorium.com:
The SERENDIP IV instrument can analyze 168 million channels simultaneously! The frequencies around 1420 MHz are especially interesting to look at because of their proximity to the "water hole." Also, because of the importance of this part of the spectrum to radio astronomy, by international agreement no one is allowed to produce any broadcasts between 1420 MHz and 1427 MHz. Because of this ban, it's an especially quiet part of the spectrum.

RE: "Water Hole", From Wikipedia:
The term was coined by Bernard Oliver. The combination of hydrogen and hydroxyl yields water (the "water" part of the name); the "hole" part refers to the sudden drop in radio noise within this band.