The placement of a Low Noise Amplifier (LNA) and a Software Defined Radio (SDR) receiver such as the SDRplay RSPduo is heavily influenced by the signal loss that occurs in coaxial cables, especially at higher frequencies. Coaxial cables (e.g., RG-58, RG-213, or LMR-400) exhibit increasing attenuation as frequency rises, which means that weak signals from your antenna can be significantly degraded before they reach your receiver if the cable run is long. For example, RG-58 can lose over 6 dB per 100 feet at 100 MHz and much more at higher frequencies. This attenuation not only reduces the signal strength but also worsens the signal-to-noise ratio (SNR), as the thermal noise picked up by the cable itself is added to the signal. For sensitive SDR applications, such as weak-signal VHF/UHF reception or radio astronomy, this loss can be detrimental. Placing the LNA as close to the antenna as possible ensures that the weak signals are amplified before encountering any significant cable loss, preserving both signal strength and SNR. If the LNA and SDR are located far from the antenna and connected via a long coax, the LNA will amplify both the attenuated signal and the noise picked up along the cable, which cannot recover the original SNR. Therefore, minimizing pre-LNA cable length is crucial for optimal SDRplay RSPduo performance.
The noise figure (NF) of the receiving system determines how well weak signals can be detected above the system's inherent noise. The overall system noise figure is dominated by the first active device in the signal path—the LNA, if placed immediately after the antenna. If the LNA is mounted at the antenna, it sets the noise floor for the entire system, and subsequent losses (such as cable attenuation) have a much smaller impact on the total noise figure. This configuration maximizes sensitivity and dynamic range, which are critical for applications like weak-signal DXing or spectrum monitoring with the SDRplay RSPduo. Conversely, if the LNA is located after a long coax run, the cable loss is added to the system's noise figure before any amplification occurs, significantly degrading sensitivity. For example, a 3 dB loss in cable before the LNA effectively doubles the noise figure, which can mean the difference between detecting a signal and missing it entirely. Modern SDRs like the RSPduo have low internal noise figures, but to fully exploit their capabilities, the front-end noise figure must be kept as low as possible by placing the LNA at the antenna. This is especially important at VHF and above, where atmospheric noise is lower and system noise becomes the limiting factor.
While placing the LNA and SDR right at the antenna is ideal for performance, there are practical considerations. LNAs require power, which can be supplied via bias-tee arrangements over the coax or via separate lines, adding complexity. The SDRplay RSPduo itself is typically USB-powered, so if you wish to mount the SDR at the antenna, you’ll need to provide USB data connectivity (often via active USB extenders or fiber) and weatherproof housing. Additionally, placing the SDR and LNA at the antenna can expose them to environmental hazards (moisture, temperature extremes) and possible electromagnetic interference (EMI) from nearby equipment, necessitating careful shielding and grounding. If the SDR is kept indoors and only the LNA is at the antenna, you still benefit from improved SNR, but must manage LNA power and possibly remote control. However, with long coax runs, signal degradation and noise pickup become the primary concerns, often outweighing the logistical challenges of remote mounting. For the SDRplay RSPduo, which supports high dynamic range and dual-tuner operation, maximizing front-end performance is key to leveraging its full capabilities, making a mast-mounted LNA (and possibly SDR) the preferred choice for demanding applications.