The 22 GHz astrophysical maser is a water (H₂O) maser.
The principal water maser transition is:
- 22.23508 GHz (22,235.080 MHz)
This corresponds to the 6₁₆ → 5₂₃ rotational transition of water vapour.
Water masers are found in:
- Star-forming regions (e.g. Orion KL, W49N)
- The envelopes of evolved stars
- The accretion disks of some active galactic nuclei (megamasers)
For comparison:
- Neutral hydrogen (H I): 1.420405752 GHz
- OH masers: 1.612–1.720 GHz and several excited-state transitions around 4.7 GHz, 6.0 GHz, and 13.4 GHz
- Water (H₂O) maser: 22.23508 GHz
If your interest is 22 GHz radio astronomy, you’re looking for the 22.23508 GHz water vapour line, rather than an OH transition.
If you’re looking for a 22 GHz filter for amateur radio astronomy (the 22.235 GHz water vapour spectral line), there are several possibilities depending on what you need:
- Waveguide band-pass filters – the standard choice at 22 GHz. They are typically built in WR-42 waveguide (18–26.5 GHz) and provide low insertion loss with excellent rejection outside the passband.
- Cavity filters – available commercially for the K-band and can be designed with bandwidths ranging from a few MHz to several hundred MHz.
- Microstrip filters – smaller and cheaper but generally have higher losses than waveguide filters.
For radio astronomy, a filter centred on 22.235 GHz with a bandwidth of 50–200 MHz is often suitable, depending on your receiver and whether you want to reject nearby interference.
Manufacturers that may be able to offer custom or standard K-band filters include:
- Pasternack
- Fairview Microwave
- QuinStar Technology
- Eravant
- ****WTMicrowave**** – I have purchased some excellent and very keenly priced (cheap but well made) cavity filters from this company for hydrogen line RA in past, and am currently exploring what options they can provide, if any, for 22 GHz.