Question E2D03:
Which of the following digital modes is designed for EME communications?  

Choices:  
A: MSK144  
B: PACTOR III  
C: WSPR  
D: Q65    

Correct Answer is  D: Q65  

Q65 is correct because it was specifically designed (by K1JT, WSJT-X) for extremely weak-signal VHF/UHF/SHF work including EME. It handles Doppler shift, libration fading, and very high path loss, and it supersedes JT65/QRA64 for EME use.

Why the other choices are incorrect:
- MSK144: Optimized for meteor scatter bursts on VHF (short, fast bursts), not for continuous, high-loss EME paths.
- PACTOR III: HF ARQ data mode for robust point-to-point data (e.g., email over HF). Not a weak-signal EME mode and not suited to large Doppler/fading.
- WSPR: Beaconing/propagation-reporting mode with ultra-low symbol rate; not intended for two-way QSOs and not optimized for EME’s Doppler and libration characteristics.

Useful formulas and rules of thumb:
- Free-space path loss:
  $$\mathrm{FSPL(dB)} = 32.44 + 20\log_{10}(f_{\mathrm{MHz}}) + 20\log_{10}(d_{\mathrm{km}})$$
  Rules of thumb: doubling distance ≈ +6 dB; doubling frequency ≈ +6 dB.
- EME path loss (2 m band) is roughly:
  $$\text{Total EME loss at }144\ \text{MHz} \approx 250\ \text{dB}$$
  (includes very long path and lunar reflection loss).
- Doppler shift due to lunar motion:
  $$\Delta f \approx \frac{v}{c}\,f$$
  Example: at $f=144\ \text{MHz}$ and $v \sim 1\ \text{km/s}$, $\Delta f$ is a few hundred hertz—Q65 is designed to cope with this.