Question E6E05:
Which of the following noise figure values is typical of a low-noise UHF preamplifier?  

Choices:  
A: 0.5 dB  
B: -10 dB  
C: 44 dBm  
D: -20 dBm    

Correct Answer is  A: 0.5 dB  

To recognize a typical low-noise UHF preamp noise figure, remember that good modern LNAs have noise figures well under 1 dB.

Why A (0.5 dB) is correct:
- A high-quality UHF low-noise preamplifier commonly has a noise figure around 0.3–1.0 dB. A value of 0.5 dB is realistic and indicates excellent performance.

Why the others are incorrect:
- B: -10 dB — Noise figure cannot be negative. By definition:
  $$NF_{\mathrm{dB}} = 10\log_{10}(F),\quad F=\frac{\mathrm{SNR_{in}}}{\mathrm{SNR_{out}}} \ge 1 \Rightarrow NF_{\mathrm{dB}} \ge 0$$
- C: 44 dBm — This is a power level (absolute, referenced to 1 mW), not a noise figure.
- D: -20 dBm — Also a power level, not a noise figure.

Useful formulas and rules of thumb:
- Noise figure and noise factor:
  $$NF_{\mathrm{dB}} = 10\log_{10}(F)$$
- Relation to equivalent noise temperature:
  $$T_e \approx 290\,(F-1) = 290\left(10^{NF_{\mathrm{dB}}/10}-1\right)$$
  For $NF = 0.5\ \mathrm{dB}$:
  $$F = 10^{0.5/10} \approx 1.122,\quad T_e \approx 290\times0.122 \approx 35\ \mathrm{K}$$
- Cascaded systems (shows why a low-NF, high-gain first stage matters):
  $$F_{\text{total}} = F_1 + \frac{F_2-1}{G_1} + \frac{F_3-1}{G_1G_2} + \cdots$$

Rule of thumb: For UHF weak-signal work, an LNA with NF < 1 dB (around 0.5 dB) is considered very good.