In many radio frequency (RF) applications, particularly those involving weak signals or long cable runs, the placement of a low-noise amplifier (LNA) and software defined radio (SDR) receiver such as the ADALM-Pluto SDR is a critical design consideration. The two common configurations are: (1) mounting the LNA and SDR receiver (Pluto) directly at the antenna, and (2) separating the LNA and SDR, connecting them via a long coaxial cable. Each approach has trade-offs regarding noise figure, signal loss, dynamic range, and practical deployment. Recent discussions and measurements in the SDR community (2022–2024) provide insight into these factors.
Placing the LNA directly at the antenna is widely regarded as the optimal setup for maximizing receiver sensitivity, especially for weak-signal applications. The primary reason is that the first stage noise figure dominates the overall system noise performance. By amplifying the signal before it experiences any significant cable loss, the LNA ensures that the signal-to-noise ratio (SNR) is preserved as much as possible. Losses in coaxial cables, particularly at higher frequencies (such as 1 GHz and above, where the Pluto SDR operates), can be substantial—even premium cables like LMR-400 can introduce losses of 1–2 dB per 10 meters. If the LNA is placed after the cable, all signal and noise picked up by the cable are amplified equally, and the initial SNR reduction due to cable loss cannot be recovered. Recent measurements (see RTL-SDR LNA Placement Guide, 2023) confirm that for every 3 dB of cable loss before the LNA, the received SNR is halved, which can be detrimental for weak signals. Additionally, mounting the Pluto SDR at the antenna can minimize radio-frequency interference (RFI) from long cable runs, further improving performance. However, this setup may require weatherproof enclosures and robust power solutions, which can add complexity.
Deploying the LNA and SDR receiver with a long coaxial cable in between is sometimes necessary for practical reasons—such as when the antenna is mounted on a roof or mast, far from the radio operator or computer. In this configuration, the LNA's placement is still critical: it should be as close to the antenna as possible. If the LNA is placed after the long cable, the system noise figure increases dramatically because the cable loss directly precedes the first amplification stage. This means that not only is the signal attenuated, but the effective noise floor rises, so weak signals may become unrecoverable. Even when the LNA is mounted at the antenna, the long coaxial cable after the LNA can still introduce losses, but because the signal has already been amplified, the impact on SNR is much less severe. The Pluto SDR itself is sensitive to input signal levels, and excessive cable loss before the LNA can result in signals falling below its noise floor. Recent user experiences (see Analog Devices Forums, 2024) emphasize that for optimal performance, especially above 500 MHz, the LNA should be at the antenna and only a short cable should separate the LNA and Pluto SDR whenever possible. If a long cable is unavoidable, high-quality, low-loss cable is recommended, and the LNA should always be the first component encountered by the signal from the antenna.
In summary, the best performance with the ADALM-Pluto SDR is achieved when the LNA is mounted directly at the antenna, minimizing pre-amplification losses and preserving signal integrity. Separating the LNA and SDR with a long coaxial cable is only advisable when the LNA is still placed at the antenna; otherwise, significant degradation in SNR and sensitivity will occur. Recent community and manufacturer guidance consistently recommend antenna-mounted LNAs for weak-signal work, with careful attention to cable quality and length if the Pluto SDR itself cannot be co-located. These practices are essential for applications such as satellite reception, weak signal propagation studies, and other scenarios where every decibel of SNR matters.