One of the most critical factors affecting the performance of a software defined radio (SDR) system, such as the KiwiSDR, is the amount of signal loss that occurs in the coaxial cable between the antenna and the receiver. Coaxial cables exhibit frequency-dependent attenuation, meaning that higher frequencies are attenuated more than lower frequencies. For example, a typical RG-58 coaxial cable might lose several decibels (dB) of signal per 100 feet at HF frequencies (3–30 MHz), which can be significant, especially for weak signals. If the low-noise amplifier (LNA) is located at the antenna, it amplifies the signal before any substantial cable loss occurs, preserving the signal-to-noise ratio (SNR). Conversely, if the LNA and SDR are located at the far end of a long coaxial run, any losses in the cable will reduce both the signal and noise, but the SNR will not be improved by amplification after the loss. This is a fundamental concept in radio receiver design: amplifying a weak signal after it's already been attenuated by cable losses cannot restore the original SNR. Therefore, for optimal performance, particularly in installations where the antenna is far from the receiver (such as remote or outdoor antennas), mounting the LNA directly at the antenna is highly recommended to minimize the impact of cable losses on overall system sensitivity.
The noise figure (NF) of the receiver system is another crucial performance factor, especially for weak-signal reception with the KiwiSDR. The noise figure quantifies how much noise the receiver adds to the incoming signal, and a lower NF indicates better sensitivity. When the LNA is placed at the antenna, it amplifies the desired signal and the noise present at the antenna, but since the LNA typically has a very low noise figure, the overall system noise figure is dominated by the LNA rather than by the losses and noise added by the coaxial cable or subsequent receiver stages. If the LNA and SDR are separated from the antenna by a long cable, the cable’s loss degrades the signal before amplification, and the noise figure of the cable plus the LNA/SDR combination increases. According to Friis’ formula for cascaded noise figure, early-stage losses are especially detrimental. For the KiwiSDR, which is often used for weak signals in noisy HF environments, minimizing the system noise figure by placing the LNA at the antenna can significantly improve the ability to receive weak signals. This is particularly relevant for longwave and shortwave reception, where atmospheric and man-made noise can already be high, and every dB of additional system noise can make a significant difference in detectable signal levels.
While the technical advantages of mounting the LNA at the antenna are clear, there are also practical considerations to keep in mind. Powering the LNA at the antenna location may require bias-T power injection over the coax, or a separate power line, which can complicate installation but is often manageable. Weatherproofing and physical protection for the LNA are also important, especially in outdoor environments. For the KiwiSDR, which is commonly used by hobbyists and researchers for continuous, remote operation, reliability and ease of maintenance are paramount. Some users may opt to place the KiwiSDR itself close to the antenna and use network cabling (which is less lossy and more practical over long distances) to bring data back to the operator location, especially since the KiwiSDR is network-enabled. In summary, for the best performance, especially in weak-signal scenarios or with long cable runs, placing the LNA at the antenna is highly recommended. If only short coaxial runs are used, or if practical constraints make antenna-side amplification difficult, some performance may be sacrificed, but the system can still function adequately for stronger signals. Ultimately, the choice should be guided by the specific installation environment, signal requirements, and maintenance considerations.