Which of the following noise figure values is typical of a low-noise UHF preamplifier?

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A good low-noise UHF preamplifier adds only a few tenths of a decibel of noise, so 0.5 dB is the plausible figure. Noise figure compares an amplifier with an ideal noiseless one and therefore cannot be negative, and values in dBm state power rather than noise figure at all.