Low-noise amplifier (LNA)

Every stage in a receiver adds noise, but not equally. By the Friis formula, each stage’s noise is divided by the total gain ahead of it. So the first amplifier’s own noise counts in full, and once it has amplified the signal, everything after it (cable, filters, the SDR) matters much less. That first amplifier is the LNA, and it is the single most important part of a hydrogen-line receiver after the antenna.

Two numbers matter: its noise figure, which should be as low as you can afford, and its gain, which should be high enough to bury whatever comes after it.

Why gain matters too

Take a chain of: LNA with a 1.0 dB noise figure, then 3 dB of coax, then an SDR with a 6 dB noise figure. (Example values, not a specific product.) The receive-chain noise calculator gives:

LNA gain	LNA	Coax	SDR	Receiver total
10 dB	75.1 K	28.9 K	172.5 K	276 K
20 dB	75.1 K	2.9 K	17.2 K	95 K
30 dB	75.1 K	0.3 K	1.7 K	77 K

With only 10 dB of gain, the SDR’s noise still dominates. At 30 dB the receiver is almost exactly as good as the LNA alone. Past that point, more gain stops helping and starts risking overload, so two stages of about 20 dB each, or one good 30 to 40 dB stage, is the usual answer.

Why it goes at the feed

Put the same 3 dB of cable in front of the LNA instead and the receiver temperature jumps to about 456 K, nearly five times worse. Loss ahead of the first amplifier adds its full noise and also divides your signal, and no amount of later gain gets that back. LAB-011 works this through with real cable lengths.

Filtering and power

L-band is crowded with strong signals near the hydrogen line (navigation, satellite phones, cellular; see RFI). A wideband LNA amplifies all of them, and a strong one can push the SDR into compression across the whole band. A SAW-filtered LNA centred on 1420 MHz rejects most of that before it reaches the SDR. That’s why the chain in LAB-006 uses one.

Most small LNAs are powered over the coax by a bias tee: the SDR (or an inline injector) puts DC on the centre conductor and the LNA takes it off. If your SDR’s bias tee is off, the LNA is just a lossy connector. It’s the first thing to check when you see no line.