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Radio Astronomy (1400-1427 MHz)
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This band is one of the most important bands for radio astronomy, since it encompasses a fundamental spectral line of hydrogen, which is by far the most prevalent element in the Universe.
Neutral (non-ionized) hydrogen naturally emits (or absorbs) at the frequency of 1420.4058 MHz due to a "spin flip" transition in energy states. Referred to as the HI line (pronounced "H-one line"), it can be observed throughout the cosmos and is heavily used for probing conditions in our own Galaxy and in other galaxies.
Due to relative motions between us and the cosmic sources being observed, the HI line can be seen at frequencies other than its rest frequency of 1420.4058 MHz. In fact, it is believed that neutral hydrogen can be observed at such great distances (and therefore many billions of years in the past), that its signal would be redshifted to 100 MHz or even below as a result of the expansion of the Universe. The allocated range, 1400 - 1427 MHz, therefore encompasses only a small portion of the frequency range that is needed to observe HI throughout the Universe.
The line is sometimes also referred to as the 21 cm line, referring to its rest wavelength.
HI observations are made at many radio astronomy observatories worldwide.
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Frequency Bands |
| Band | Use | Service11 | Table |
| 1400 - 1427 MHz | Neutral hydrogen (HI) | Radio Astronomy | - |
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