Also known as: preamplifier, preamp, RF preamp
A preamplifier (preamp) is a gain stage placed early in a receive chain — ideally right at the antenna — to lift weak signals above the noise added by everything downstream, improving the receiver’s overall sensitivity.1 In RF practice the term is used almost interchangeably with low-noise amplifier: a preamp is an LNA, described by the job it does in the signal chain rather than by a different circuit.
Overview
The value of a preamp comes straight from the cascade noise formula (Friis): the first amplifier’s noise figure dominates the whole receiver’s noise, and its gain divides down the noise contribution of every stage after it. Put a low-noise gain block first and the rest of the chain — mixers, filters, lossy coax, the SDR itself — barely moves the noise budget. Put it last and it does almost nothing.
How it works
Two numbers define a preamp: its gain (how much it boosts the signal) and its noise figure (how much noise it adds relative to a perfect amplifier). Because it sits first, you want the lowest noise figure available and just enough gain to make later stages negligible — typically 10–20 dB. Excess gain is counter-productive: it eats into the receiver’s headroom and can push strong signals into overload, generating intermodulation. A good preamp is therefore a balance of low noise, adequate gain, and enough linearity to survive the strongest signals present.
Mounting matters as much as the device. A mast-mounted preamp placed before the coax recovers cable loss that would otherwise degrade sensitivity, and it is commonly powered up the same coax through a bias tee so no separate power cable is needed.
Relevance to SDR
Preamps are a staple of SDR reception. Many popular receivers — RTL-SDR dongles, Airspy, SDRplay — have modest front-end noise figures, so an external low-noise preamp near the antenna can noticeably improve weak-signal reception, especially at VHF/UHF with long feed lines. The caveat is overload: an SDR in a strong-signal environment (near broadcast FM or paging transmitters) can be made worse by a preamp that amplifies everything into the tuner’s compression region. Filtering before the preamp, or using it only where the band is genuinely quiet, is the usual remedy.
For GopherTrunk, a preamp is upstream hardware, not part of the software. It affects the quality of the I/Q stream GT receives — a better front-end noise figure means cleaner symbols and fewer errors for the decoders — but GopherTrunk neither controls nor requires one; it works with whatever signal the SDR delivers.
Sources
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Preamplifier — Wikipedia, on preamplifiers as early gain stages and their role in setting system noise performance. ↩