Also known as: low-noise amplifier, LNA
A low-noise amplifier (LNA) boosts a weak antenna signal early in the receive chain, adding as little noise as possible.1 Because every later stage contributes its own noise, amplifying the signal first — before cable and mixer losses eat into it — preserves the signal-to-noise ratio and largely fixes the whole receiver’s sensitivity. An LNA is a specific kind of preamplifier: one designed so that its own noise figure is as low as the gain it provides is useful.
How it works
The reason placement matters is captured by the Friis noise formula for a chain of stages.2 The total noise factor is
F_total = F1 + (F2 − 1)/G1 + (F3 − 1)/(G1·G2) + …
The first stage’s noise factor F1 passes through untouched, but every later stage’s noise
contribution is divided by the gain that precedes it. Give the first stage enough gain
G1 and its low F1 dominates: the noise of the coax, the mixer, and the SDR’s own
ADC are suppressed by that gain. This is why the
LNA belongs at the antenna, ahead of the feedline, not bolted to the radio. Its
noise figure — noise factor expressed in dB — is the single
number that most determines how weak a signal the receiver can still recover.
An LNA is not free gain, though. It also amplifies every strong signal in its passband, and its output devices have a finite 1 dB compression point and third-order intercept. Drive it too hard and it generates intermodulation products that can bury the wanted signal — so a “better” LNA is often one with lower gain but higher linearity.
Variants
- Wideband LNA — flat gain over a broad range (e.g. 20 MHz–2 GHz). Simple and cheap, but it amplifies out-of-band offenders too, so it is usually paired with an RF filter or SAW filter preselector.
- Band-specific / filtered LNA — an integrated filter plus amplifier for one service (1090 MHz ADS-B, the 137 MHz weather-satellite band). The filter protects the amplifier from overload and is the more robust choice near strong transmitters.
- Mast-mounted amplifier (MMA) — a weatherproof LNA at the antenna, fed DC up the coax by a bias tee so no separate power cable is needed.
- GaAs / GaN pHEMT devices dominate modern low-noise designs; noise figures below 1 dB are routine at VHF/UHF.
In practice
Add an LNA only when the front end is genuinely noise-limited — long or lossy coax, a weak-signal target, or a masthead install. In a strong-signal urban environment an un-filtered LNA usually makes reception worse by pushing the SDR into overload; there, reach for a filtered LNA or an attenuator instead. Because the RTL-SDR and similar tuners already have modest noise figures, the LNA’s biggest wins come on the highest bands and longest runs, where cable loss would otherwise dominate the budget.
Relevance to SDR
An antenna-mounted LNA can meaningfully improve reception of weak signals on hobby SDRs — ADS-B at 1090 MHz, L-band satellite, and long UHF runs are classic cases — provided you guard against overload from strong nearby transmitters. GopherTrunk is downstream of the front end: it decodes whatever IQ the SDR delivers and does not control the LNA, but a well-chosen LNA raises the in-channel SNR that GopherTrunk’s demodulators see, which is the difference between a locked control channel and a dropped one. See the DSP notes on rate-invariance — the decode path is only as good as the samples handed to it, and the LNA is where that quality is won or lost.
Sources
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Low-noise amplifier — Wikipedia, on LNAs, noise figure, and their role in setting receiver sensitivity. ↩
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Friis formulas for noise — Wikipedia, deriving how each cascaded stage’s noise is divided by preceding gain, the reason the LNA goes first. ↩