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We are building a multi-stage LNA for a TR module we are building in the 144MHz and 440MHz range. This LNA we are considering (Qorvo QPL9547) provides about 22dB gain in the 2m and about 27dB in the 440MHz according to the Gain vs Frequency graph.
A LNA amplifies signal and noise. It is of some use only if:
Many VHF/UHF receivers already have sufficiently good noise performance that one or both of these things is not true, and so adding an LNA will not improve performance. Typically, an LNA improves performance only after the antenna is very directional, and in a quiet environment. A not-so-directional antenna, or one in any urban or semi-urban environment, will be subject to high RF noise, rendering the first point untrue and thus an LNA of no benefit.
Moreover, if an LNA is potentially of some benefit, it's not just because it has high gain, but because it has low noise. The "LN" in "LNA" is for "low noise", after all. If it was just a question of gain maybe it would be a "HGA", or "high gain amplifier".
How much gain is useful, and how low the noise must be depends on several variables. See How can I calculate the effects of an LNA, antenna gain, etc. on noise performance? for details.
An earlier Answer mentioned LNAs would be “of some use only if ... the noise power from the antenna is higher than the noise floor of your receiver” (quote). That statement appears in need of some clarification. If the receiver, on its own, has a high noise figure, or is preceded by a (long) lossy feedline (which contributes/adds to the system noise figure) then it is useful to place an LNA close to the antenna feed point, to reduce the receive-chain’s (own) system noise figure. (This is common practice in satellite comms/broadcasting with LNA (and converter) inside the feed of TV parabolic dish antennas at home.) About high noise power intercepted by the antenna, an LNA cannot do anything, except amplifying this noise in the same way as it amplifies the intended signal intercepted by the antenna.
A second LNA behind the first LNA does usually not significantly improve the system noise figure and consequently also not the (S/N) ratio. However, additional amplification might still be essential in low-signal environment to realize the system gain necessary to reach the absolute signal level (power in dBm, or rms voltage in μV) which satisfies the absolute input signal level required by the (given) receiver. For such second amplifier its own noise figure is usually not critical for the system noise figure.
Other parameters like signal bandwidth; power level of the signal from the antenna; relative receiver sensitivity (S/N); etc., are important parameters in a typical RF receiver cascade system trade-off, which is a perfect application for spreadsheet tools of which numerous are freely available.
The MS Excel macro-enabled tool “Cascade” shown below, can be downloaded here. It includes comments to make the tool easy-to-use and self-explanatory; it is useful especially also for ham radio applications. The tool has been validated and is maintained regularly. User feedback and comments are welcome to further develop the tool.
