In the United Kingdom, the L-band around the 1420 MHz hydrogen line is formally recognised and protected for passive scientific use, meaning emissions by other services are prohibited or very tightly restricted to avoid harmful interference to radio astronomy and Earth exploration satellite passive sensing.
📡 UK Protected Allocation for 1420 MHz
- The UK’s national frequency allocation tables list 1400 MHz to 1427 MHz as a primary allocation to radio astronomy, Earth exploration-satellite (passive) and space research (passive) services. Within this range, transmissions are generally forbidden and the band is protected to enable sensitive scientific observations such as the 21 cm hydrogen line.
- More precisely, the hydrogen line is at 1420.40575 MHz; so having protection from 1400 MHz up to 1427 MHz gives a total protected span of about 27 MHz around that frequency.
🧠 Context
- This protected band is used for radio astronomy observations, including mapping neutral hydrogen in the Galaxy and other astrophysical research, and also for passive Earth observation instruments that must not have interfering transmissions in-band.
- Immediately above that, from about 1427 MHz upwards (e.g., 1427–1452 MHz), other services such as fixed/mobile are allocated in the UK, often under conditions that coordination and limits on out-of-band emissions are applied to protect the adjacent passive band.
In summary:
✅ Approx. 1400–1427 MHz (~27 MHz) in the UK is formally protected for passive scientific use including radio astronomy around the 1420 MHz hydrogen line.