Explain why the low-noise amplifier of receiver system is laced at the antenna end of the feeder cable?

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written 9.5 years ago by

teamques10 ★ 70k

Low-noise amplifier (LNA) is an electronic amplifier used to amplify possibly very weak signals. It is usually located very close to the detection device to reduce losses in the feedline. This active antenna arrangement is frequently used in microwave systems like GPS, because coaxial cable feedline is very lossy at microwave frequencies, e.g. a loss of 10% coming from few meters of cable would cause a 10% degradation of the signal-to-noise ratio (SNR).
An LNA is a key component which is placed at the front-end of a radio receiver circuit. Using an LNA, the effect of noise from subsequent stages of the receive chain is reduced by the gain of the LNA, while the noise of the LNA itself is injected directly into the received signal.
Thus, it is necessary for an LNA to boost the desired signal power while adding as little noise and distortion as possible, so that the retrieval of this signal is possible in the later stages in the system. A good LNA has a low NF (e.g. 1 dB), large enough gain (e.g. 20 dB) .Further criteria are operating bandwidth, gain flatness, stability and input and output voltage standing wave ratio (VSWR).
For low noise, the amplifier needs to have a high amplification in its first stage. Therefore JFETs and HEMTs are often used. They are driven in a high-current regime, which is not energy-efficient, but reduces the relative amount of shot noise.
In a satellite communications system, the ground station receiving antenna will connect to a LNA. The LNA is needed because the received signal is weak. The received signal is usually a little above background noise. Satellites have limited power so they use low power transmitters. The satellites are also distant and suffer path loss.
The LNA boosts the antenna signal to compensate for the feedline losses going from the (outdoor) antenna to the (indoor) receiver. In many satellite reception systems, the LNA includes a frequency block down converter that shifts the satellite downlink frequency (e.g., 11 GHz) that would have large feedline losses to a lower frequency (e.g., 1 GHz) with lower feedline losses. The LNA with down converter is called a low-noise block down converter (LNB).

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