Question E3A13:
Which of these emission modes is best for auroral propagation?  

Choices:  
A: CW  
B: SSB  
C: FM  
D: RTTY    

Correct Answer is  A: CW  

For auroral propagation, CW is best because aurora causes very rapid flutter, phase distortion, and Doppler spreading that severely garble complex or wideband signals. A narrow, simple on/off keyed signal is most resilient and can still be copied by ear even when it sounds “raspy.”

Why A (CW) is correct:
- CW is very narrowband (typically 100–500 Hz), so it preserves usable SNR under noise and frequency spreading.
- CW’s simple envelope tolerates fast fading and distortion; operators can still recognize the auroral “buzz” tone and copy characters.

Why the others are incorrect:
- SSB: Wideband (~2.4 kHz) and dependent on waveform fidelity. Auroral Doppler spread and phase distortion make speech unintelligible (“auroral warble”).
- FM: Wide deviation and capture/threshold behavior perform poorly with frequency/phase distortion and selective fading from aurora. The result is severe distortion or complete loss.
- RTTY: Two closely spaced tones are smeared by Doppler/spread and flutter, causing high symbol errors. Decoders lose lock easily.

Useful rules of thumb and formulas:
- Narrower bandwidth improves SNR in noise-limited conditions:
  $$N = k T B \quad \Rightarrow \quad \text{SNR} \propto \frac{1}{B}$$
- Relative SNR improvement from reducing bandwidth:
  $$\Delta \text{SNR}_{\text{dB}} \approx 10 \log_{10}\!\left(\frac{B_{\text{wide}}}{B_{\text{narrow}}}\right)$$
  Example: comparing SSB (2400 Hz) to CW (300 Hz) gives about
  $$10 \log_{10}(2400/300) \approx 9 \text{ dB}$$
  This extra margin helps overcome auroral flutter and Doppler spreading, making CW the preferred mode.