By Flavio Falcinelli
The official classification of the frequency bands of the radio spectrum is shown in the following table.
The Earth's atmosphere restricts the use of the frequencies usable for radio astronomical observations from the ground, since it behaves as a true barrier to the electromagnetic radiation coming from space. In fact, the direct measurement of the cosmic radiation is limited to two "windows" of the electromagnetic spectrum, one comprised between about 0.3 and 0.8 micrometers (visible window, with amplitude of about one octave) and one between about 1 centimeter and 1 meter wavelength (radio window, with amplitude greater than 10 octaves).
Classification in the frequency bands of the radio spectrum.
The "radio" window is bounded below by the shielding effects of the ionosphere (electrically charged particles that act as a reflector for radio waves), and above by molecular absorption phenomena due to water vapor and oxygen.
As can be seen from the images, the range of useful radio frequencies for radio astronomical observations from the ground is between about 20 MHz and 20 GHz.
The Earth's atmosphere effects are clearly visible when comparing the graphs that represent the radio "window" of the electromagnetic spectrum from the ground view (left) and from a radio telescope operating in space (right).