The placement of a Low Noise Amplifier (LNA) in a receiving chain significantly affects the overall system sensitivity and noise performance, especially in setups involving Software Defined Radios (SDRs) such as the FUNcube Dongle Pro+. The fundamental principle is that the LNA should be positioned as close to the antenna as possible. This proximity is critical because any signal loss incurred before the LNA—such as from a coaxial cable—directly degrades the signal-to-noise ratio (SNR) of the entire system. Coaxial cables, even high-quality low-loss variants, introduce attenuation that not only reduces signal strength but also increases the effective noise figure by allowing the cable's own thermal noise to dominate before amplification occurs. By mounting the LNA at the antenna, you amplify the desired weak signals before they suffer significant loss or noise injection. This is particularly important for the FUNcube Dongle Pro+, which, while sensitive, can benefit greatly from a low upstream noise figure when receiving weak satellite signals or in environments with high cable run lengths. Thus, antenna-mounted LNAs are recommended for optimal performance, as substantiated by recent amateur radio and SDR community consensus and technical analyses.
If the LNA and SDR receiver, such as the FUNcube Dongle Pro+, are both located far from the antenna and connected via a long coaxial cable, several performance penalties arise. The primary issue is signal attenuation: coaxial cables introduce loss that is frequency-dependent and can be substantial at VHF, UHF, and higher bands. For instance, a typical RG-58 coaxial cable can lose several dB per 10 meters at 144 MHz, and even more at higher frequencies. This attenuation not only weakens the signal but also elevates the system noise figure, as the weak signal is mixed with cable-generated noise before reaching any amplification stage. The result is a notable reduction in receiver sensitivity, making it harder to detect weak signals—an important factor in satellite communications and weak-signal amateur radio work. Furthermore, the SDR’s internal LNA, like that in the FUNcube Dongle Pro+, cannot compensate for signal and SNR loss that occurs before it. Modern research and user experience reports (2022–2024) on SDR forums and technical blogs consistently demonstrate that pre-LNA cable loss is a dominant factor in overall system degradation. Therefore, it is generally advised to avoid long coaxial runs before the first LNA when using SDR receivers, especially for applications requiring high sensitivity.
For users of the FUNcube Dongle Pro+, the optimal configuration is to mount a high-quality LNA directly at the antenna feed point, followed by the coaxial cable run to the SDR receiver. This setup ensures that the LNA amplifies the weak RF signals before any significant cable loss can occur, preserving both signal strength and SNR. Powering the LNA can typically be accomplished with bias-tee arrangements, which are supported by the FUNcube Dongle Pro+ and many modern LNAs. In cases where mounting the LNA at the antenna is impractical (e.g., due to environmental exposure, power considerations, or physical constraints), users should minimize coaxial run length and use the lowest-loss cable feasible for their frequency of interest. In summary, the consensus from recent technical literature and the SDR user community is clear: the best performance, especially with sensitive SDRs like the FUNcube Dongle Pro+, is achieved when the LNA is placed at the antenna. This approach minimizes noise, maximizes sensitivity, and is particularly crucial for weak signal and satellite reception scenarios.