Hum,
the wavelength of 21 cm is very empty of background noise.
It`s the idea station to listen into . . . radiation with a very specific frequency--1420 million cycles per second is the natural frequency of a precession of the spin of an electron as it encircles the atomic nucleus of hydrogen, the dominant material in the universe.
It is, by a factor of at least 1000, the most prominent radio frequency in the galaxy. Moreover, it falls precisely in the window of frequencies that, like visible light, can be transmitted and received through an atmosphere capable of supporting life. . . . IT is the universal ILS beacon.
But when we tune into it, we tune slightly above that frequency, and listen to an extremely narrow range of radio frequencies--the so-called "water hole" between the hydrogen line of 1420 megahertz and the first hydroxyl line of 1662 megahertz.4
There are no other known spectral lines between these frequencies, suggesting that civilizations would use this range for communication because it is a very quiet channel.
The "quietness" of this range of frequencies indirectly implies what it is that SETI researchers are looking for: noise. Beyond the omnipresent cosmic background radiation remaining from the Big Bang, there is almost no natural emission in this range, and even the background radiation is essentially at a constant level. Even low-power transmitters exceed this natural background.
Also, there are practical limitations to our ability to receive many of these frequencies, especially from our protected position beneath the shield of the earth's atmosphere.
Frequencies below 15 Mhz or so, are rarely used due to absorption of these waves by the ionosphere (an atmosphere). At the upper end of the frequency range, limitations are imposed by the technology needed to receive signals with such tiny wavelengths.
Most amateur radio telescopes fall between 18 Mhz and 10,000 Mhz.
