The placement of a Low Noise Amplifier (LNA) and a Software Defined Radio (SDR) receiver, such as the SDRplay RSP1A, is a crucial consideration in radio systems, especially when long coaxial cables are involved. The key factors influencing system performance include noise figure, signal attenuation, and potential interference. With the increasing popularity of SDRs and compact LNAs, understanding whether these components should be mounted directly at the antenna or separated by a coaxial cable is essential for optimizing signal quality and sensitivity.
The most significant performance factor when using an LNA with an SDR is the overall system noise figure. Placing the LNA as close as possible to the antenna is generally recommended. This is because coaxial cables, even high-quality ones, introduce signal loss before the signal reaches the LNA or SDR. Any loss before the first amplification stage not only reduces the signal amplitude but also increases the system's effective noise figure, degrading sensitivity. According to the Friis formula for cascaded noise figure, loss before the LNA is particularly detrimental since it attenuates both signal and noise, but the LNA's gain cannot recover the lost signal-to-noise ratio (SNR). For the SDRplay RSP1A, which has a typical receiver noise figure around 7-9 dB, a high-quality LNA with a noise figure below 1 dB can dramatically improve weak signal reception if placed at the antenna. If the LNA is instead placed after a long coaxial run, the cable's loss (often 3-10 dB or more over tens of meters) will raise the noise floor and reduce the effectiveness of the LNA, since the SNR loss from the cable cannot be recovered.
Coaxial cable loss is a critical factor in system design. As frequency increases, so does the attenuation per unit length of typical coaxial cables (e.g., RG-58 or RG-213). For VHF and UHF applications, losses can be substantial, with 100 feet of RG-58 causing over 6 dB of loss at 150 MHz, and even more at higher frequencies. This loss not only reduces the received signal strength but also degrades the noise performance of the entire receiving system. If the LNA is mounted at the antenna, it amplifies the weak signal before any significant loss occurs, preserving the SNR. The amplified signal can then travel through the coaxial cable to the SDR receiver with minimal additional degradation, since the LNA's gain overcomes the cable loss. In contrast, if the LNA is mounted near the SDR, after a long cable, the signal arrives already weakened and with a worsened SNR, and the LNA cannot recover what is lost. Modern installations often use bias tees to power the LNA through the coax, making antenna-end mounting practical. For the SDRplay RSP1A, which can supply bias voltage, this setup is convenient and highly recommended for optimal performance.
In summary, for the best system sensitivity and lowest noise figure when using the SDRplay RSP1A SDR and an LNA, mounting the LNA as close as possible to the antenna is strongly recommended, especially when long coaxial cable runs are necessary. This configuration ensures that the weakest signals are amplified before any significant loss or noise is introduced by the cable. Modern LNA designs are compact, efficient, and can be remotely powered, making antenna-side installation feasible in most scenarios. Separating the LNA and SDR by a long coaxial cable without placing the LNA at the antenna will result in degraded performance, particularly at higher frequencies and longer cable lengths. Therefore, for optimal results, always prioritize placing the LNA at the antenna and use a bias tee for remote powering, leveraging the RSP1A's capabilities for a high-performance SDR receiving system.