At 1420 MHz (the hydrogen line), the lowest system noise temperature an amateur can realistically achieve depends on the quality of the antenna, feed, low-noise amplifier (LNA), and how much unwanted ground noise enters the system.
A radio telescope’s total system temperature is approximately:
T_{sys} = T_{sky} + T_{ground} + T_{feed} + T_{LNA}
For a well-designed amateur hydrogen-line system:
Component Typical Temperature
Cold sky background 3-10 K
Galactic background (away from Milky Way) 1-5 K
Feed losses 5-20 K
Good amateur LNA 20-40 K
Ground spillover 5-30 K
Total 35-100 K
The best amateurs have achieved:
50-80 K system temperatures routinely.
30-50 K with excellent feed illumination and very low-noise LNAs.
Around 20-30 K is exceptionally difficult and usually requires professional-style cryogenic techniques.
Modern hydrogen-line LNAs based on devices such as the SPF5189Z RF amplifier or low-noise pHEMT transistors often have noise figures of 0.3-0.5 dB, corresponding to:
T = 290(F-1)
For example:
0.5 dB NF → about 35 K
0.3 dB NF → about 21 K
0.2 dB NF → about 14 K
Using the relation:
T=290(F-1)
where is the linear noise factor.
For someone operating a hydrogen-line dish at the Lichfield Radio Observatory level of sophistication, with careful feed design and a mast-mounted LNA, a realistic target would be:
Excellent amateur system: 40-60 K
Very good amateur system: 60-100 K
Basic amateur system: 100-200 K
A useful benchmark is that many successful amateur hydrogen-line telescopes detect the Galactic plane and hydrogen emission with system temperatures around 80-150 K.