When assembling a high-performance receiving setup using a software-defined radio (SDR) such as the RX-888 and a low-noise amplifier (LNA), a crucial design consideration is the placement of the LNA relative to the antenna and the SDR receiver. Specifically, operators often ask whether it is better to mount the LNA (and possibly the SDR itself) directly at the antenna, or if it is acceptable to separate the LNA and SDR by a long coaxial cable run. The answer to this question involves understanding several key performance factors, including signal loss, noise figure, and the impact of cable characteristics on overall system sensitivity. This discussion provides an up-to-date and detailed analysis based on recent sources and best practices.
The most significant factor influencing the decision is the loss introduced by the coaxial cable between the antenna and the receiver. Coaxial cables introduce attenuation that increases with frequency, length, and cable type. For example, at VHF or UHF frequencies, even high-quality coaxial cables such as LMR-400 can lose several dB per 100 feet. This loss not only reduces the signal strength but, more importantly, degrades the system noise figure. The noise figure of an SDR like the RX-888 is typically higher than that of a dedicated LNA. If the LNA is placed close to the antenna, it amplifies the weak signals before they encounter any significant loss, and its low noise figure dominates the overall system performance. However, if the signal must travel through a long coaxial cable before reaching the LNA, the cable's noise contribution is added directly to the system, and the LNA's benefit is greatly diminished. Thus, mounting the LNA at the antenna is almost always preferable for weak-signal or high-frequency applications, ensuring that the system's noise figure remains as low as possible.
In some scenarios, users consider placing both the LNA and the RX-888 SDR at the antenna, possibly in a weatherproof enclosure, and then running a digital or Ethernet cable back to the operating position. This approach effectively eliminates analog signal loss over coax and can be especially beneficial for remote or distributed receiving setups. However, this requires careful attention to power, data, and environmental protection requirements for the SDR and associated hardware. If only the LNA is mounted at the antenna, a short run of coax from the antenna to the LNA ensures minimal pre-amplification loss, and the amplified signal can then travel a much longer distance to the SDR with negligible impact on the system noise figure. In contrast, placing the LNA near the SDR after a long coaxial run results in substantial signal degradation and elevated noise, severely limiting weak-signal performance.
As of 2024, radio enthusiast forums, technical publications, and manufacturer recommendations continue to advocate for placing the LNA as close as possible to the antenna, with minimal unamplified cable between them. The RX-888, known for its wide bandwidth and sensitivity, benefits significantly from such a configuration, especially at VHF/UHF and higher frequencies where cable losses are more pronounced. Some modern installations leverage Power over Coax (bias-T) to supply the LNA directly from the SDR, simplifying installation. In summary, for the best performance with the RX-888 SDR, the LNA should be mounted at the antenna to preserve signal integrity, maximize sensitivity, and minimize the overall noise figure. Only in cases where cable runs are extremely short or where weak-signal performance is not critical might other configurations be considered.