Question E4C11:
Why does input attenuation reduce receiver overload on the lower frequency HF bands with little or no impact on signal-to-noise ratio?  

Choices:  
A: The attenuator has a low-pass filter to increase the strength of lower frequency signals  
B: The attenuator has a noise filter to suppress interference  
C: Signals are attenuated separately from the noise  
D: Atmospheric noise is generally greater than internally generated noise even after attenuation    

Correct Answer is  D: Atmospheric noise is generally greater than internally generated noise even after attenuation  

Correct answer: D — Atmospheric noise is generally greater than internally generated noise even after attenuation.

Why D is correct:
- On the lower HF bands (160–40 meters), the external noise (atmospheric and man-made) is very strong—often 20–40 dB above the receiver’s own noise.  
- An input attenuator reduces both the desired signal and the external noise by the same factor, so the signal-to-noise ratio (SNR) stays almost the same. But the absolute signal level at the receiver front end is reduced, preventing overload and intermodulation.

Key relationship:
- Let the attenuator have linear loss $L$ (e.g., $L=10$ for 10 dB). Let $S$ be the signal power, $N_{\text{ext}}$ external noise, and $N_{\text{rx}}$ receiver noise.
- After attenuation:
  $$S'=\frac{S}{L},\quad N'_{\text{ext}}=\frac{N_{\text{ext}}}{L}$$
  Total noise into the detector: $N' = N'_{\text{ext}} + N_{\text{rx}}$  
  SNR after attenuation:
  $$\text{SNR}'=\frac{S'}{N'}=\frac{S/L}{N_{\text{ext}}/L+N_{\text{rx}}}=\frac{S}{N_{\text{ext}}+L\,N_{\text{rx}}}$$
- If $N_{\text{ext}} \gg L\,N_{\text{rx}}$ (true on low HF), then:
  $$\text{SNR}' \approx \frac{S}{N_{\text{ext}}}=\text{SNR (no attenuation)}$$
- Meanwhile, the lower absolute level prevents front-end nonlinearity and overload.

Why the other choices are incorrect:
- A: “Low-pass filter to increase lower-frequency signals” — An attenuator is not a selective low-pass amplifier; it reduces all frequencies equally (it’s loss, not gain).
- B: “Noise filter to suppress interference” — An attenuator is passive and non-selective; it does not filter noise separately from signals.
- C: “Signals are attenuated separately from the noise” — Both signal and external noise are attenuated by the same amount; they are not separated.

Rules of thumb:
- On 160–40 m, external/band noise often dominates the receiver noise by 20–40 dB. A 10 dB attenuator usually has little to no audible SNR penalty but can greatly reduce overload.
- Use enough attenuation so that the receiver’s own noise floor remains at least about 10 dB below the external noise within your operating bandwidth. If the band is quiet (higher HF bands), attenuation may start to hurt SNR.