When deploying a software-defined radio (SDR) system such as the RTL-SDR V3, a common question arises: should the low-noise amplifier (LNA) and SDR receiver be mounted directly at the antenna, or can they be separated by a long coaxial cable? The answer depends on several performance factors, including noise figure, signal loss, interference susceptibility, and practical deployment considerations. Understanding these factors is crucial for maximizing the sensitivity and signal quality of your SDR setup. Below, we discuss the most recent insights and technical details as of 2024 to help guide your installation decisions.
The most critical performance factor when deciding where to place an LNA in an SDR system is the overall noise figure of the receiver chain. The noise figure determines how much additional noise is introduced to the received signal, directly affecting the system's sensitivity. According to the Friis formula for noise, the noise contribution of subsequent stages (such as the RTL-SDR V3 itself) is greatly reduced if a high-gain, low-noise amplifier is placed as close as possible to the antenna. This is because any signal attenuation (loss) that occurs before the first amplification stage also attenuates the desired signal, but not the noise introduced by subsequent stages. If the LNA is placed at the antenna, it amplifies both the signal and the noise present at the antenna, but crucially, it does so before any significant cable loss. When the LNA is placed at the receiver end of a long coaxial run, the cable loss attenuates the signal before it is amplified, resulting in a much higher effective system noise figure and a poorer signal-to-noise ratio. Modern RTL-SDR V3 dongles have a moderate noise figure (typically around 3-4 dB), but popular LNAs like the RTL-SDR Blog LNA or Nooelec SAWbird offer noise figures below 1 dB and gains of 20-30 dB, making them highly effective for antenna-mount applications.
Coaxial cable loss is a major consideration, especially for frequencies above 100 MHz. Standard RG-58 coax, for example, can lose 6 dB or more per 100 feet at VHF frequencies and even more at UHF. This means that if you run a long coaxial cable from the antenna to the SDR without a preamp, much of the weak signal you are trying to receive will be lost before it ever reaches the receiver. By placing the LNA at the antenna, you amplify the signal before it traverses the lossy cable, effectively overcoming the cable loss. This is why nearly all professional and advanced amateur radio installations mount LNAs at the antenna, particularly for satellite, weak signal, or microwave work. If the LNA is placed at the receiver end, any signal lost in the cable cannot be recovered, and even the best LNA cannot compensate for the loss in signal-to-noise ratio that has already occurred. This effect becomes more pronounced as frequency increases; for UHF and above, even short cable runs can significantly degrade system performance unless the LNA is placed at the antenna.
Mounting the LNA at the antenna also has implications for interference immunity and practical system design. Amplifying the signal at the antenna can help overcome not only cable loss but also local sources of electrical noise and interference that may be picked up by the cable or at the receiver location. However, this approach requires power to be supplied to the LNA at the antenna, often via bias-tee over the coaxial cable. The RTL-SDR V3 supports bias-tee power output, making this configuration straightforward. Practical considerations include weatherproofing the LNA and ensuring reliable power delivery. Longer cable runs can also act as antennas for picking up unwanted signals, so amplifying the signal before the cable helps preserve the desired signal's integrity. In contrast, placing the LNA and receiver together at the antenna may not always be practical due to space, power, or accessibility constraints, but it is almost always electrically superior. In summary, for optimal performance, especially for weak signal or high-frequency work, it is strongly recommended to mount the LNA at the antenna and use the receiver (RTL-SDR V3) at the other end of the cable. This minimizes noise figure, reduces signal loss, and improves overall system sensitivity.