The placement of a Low Noise Amplifier (LNA) in relation to the antenna and the Airspy HF+ Software Defined Radio (SDR) is crucial for optimizing the receiver’s noise figure and overall signal-to-noise ratio (SNR). When an LNA is mounted directly at the antenna, it amplifies the weak signals before they travel through any significant length of coaxial cable. This is important because coaxial cables, even high-quality types like RG-6 or LMR-400, introduce insertion loss that attenuates both the desired signal and the noise. However, the key factor is that the cable loss degrades the SNR by increasing the system noise figure, especially for weak signals. If the LNA is placed after a long cable run, the signal may already be significantly attenuated and corrupted by cable-injected noise, and the LNA can only amplify what remains, not recover lost SNR. Mounting the LNA at the antenna ensures that the first amplification stage sets the system noise figure, minimizing the impact of subsequent losses. This is particularly important for the Airspy HF+, known for its low internal noise, as preserving the best possible SNR at the input allows the SDR to perform at its highest capability. In summary, for optimal SNR and sensitivity, especially when receiving weak or distant signals, the LNA should be mounted as close to the antenna as possible, ahead of any long coaxial cable runs.
Another critical factor in deciding where to mount the LNA and SDR is the impact on dynamic range and intermodulation distortion. The Airspy HF+ is designed with a high dynamic range front end, but the overall system performance can still be affected by the placement of the LNA. When an LNA is placed at the antenna, it boosts all signals, including strong local transmitters and potential sources of interference. This can increase the risk of intermodulation products if the LNA itself is not highly linear. However, by amplifying signals before cable losses, the LNA improves the dynamic range for weak signals at the receiver. If the LNA is placed after a long cable, the dynamic range is reduced for weak signals because the SDR receives already attenuated signals, and strong signals may still be strong enough to cause overload or distortion. Therefore, for the Airspy HF+, it is generally recommended to use a high-linearity, low-noise LNA at the antenna, especially in environments with strong nearby signals. This approach maximizes the effective dynamic range and minimizes the risk of losing weak signals to cable loss or noise, preserving the SDR’s ability to distinguish between closely spaced or weak signals in the presence of strong interferers.
While technical performance favors mounting the LNA at the antenna, practical factors must also be considered. LNAs at the antenna require power, typically supplied via bias-tee through the coaxial cable or by local power sources. The Airspy HF+ supports bias-tee operation, allowing you to power compatible LNAs directly from the SDR, simplifying installation. Weatherproofing is another consideration; outdoor LNAs must be housed in enclosures that protect against moisture, temperature swings, and UV exposure to ensure long-term reliability. Cable loss is frequency-dependent: higher frequencies suffer more attenuation per unit length. For HF (0.5–30 MHz), losses are modest, but for VHF/UHF or long cable runs, they become significant. Using high-quality, low-loss coaxial cable can mitigate some losses but does not eliminate the SNR degradation caused by cable before the LNA. Therefore, for best performance with the Airspy HF+, particularly on weak or distant signals, the LNA should be mounted as close to the antenna as possible, powered via bias-tee, and protected from the elements. The SDR itself may be located indoors, connected by coax to the LNA. This setup ensures the highest sensitivity and lowest noise, allowing you to take full advantage of the Airspy HF+’s capabilities.