Question E3B06:
On which of the following amateur bands is long-path propagation most frequent?  

Choices:  
A: 160 meters and 80 meters  
B: 40 meters and 20 meters  
C: 10 meters and 6 meters  
D: 6 meters and 2 meters    

Correct Answer is  B: 40 meters and 20 meters  

Correct Answer: 40 meters and 20 meters

Why this is correct:
- Long-path is multi-hop F-layer propagation the “long way” around Earth, most common when both endpoints are near the grayline (sunrise/sunset).
- Mid-HF bands (especially 20 m, sometimes 40 m) most often have the right balance: frequency high enough to avoid excessive D-layer absorption, yet below the MUF on both legs of the path.
- 20 m ($\sim$14 MHz) frequently supports worldwide F2; 40 m ($\sim$7 MHz) can do long-path near grayline when D-layer absorption is minimized.

Why the other choices are incorrect:
- 160 m and 80 m (A): Too much D-layer absorption on the sunlit portions of the long path; these low frequencies rarely survive long daylight segments.
- 10 m and 6 m (C): Require very high MUF (solar peak conditions) for multi-hop F2; long-path can occur but is much less frequent than on 20 m/40 m.
- 6 m and 2 m (D): VHF bands seldom support F2 for global paths (2 m essentially never; 6 m only rarely). Long-path via ionosphere is exceedingly rare; propagation is mainly line-of-sight, sporadic E, or specialized modes (e.g., TEP, EME).

Useful formulas and rules of thumb:
- MUF (secant law):
  $$\text{MUF} \approx f_{oF2} \sec \theta$$
  where $f_{oF2}$ is the F2-layer critical frequency and $\theta$ is the ray’s zenith angle. Long-path needs the operating frequency below the MUF along the entire route.

- D-layer absorption increases as frequency decreases:
  $$A \propto \frac{1}{f^2}$$
  Thus, lower HF (80/160 m) suffers heavy daytime loss, making long daylight paths impractical.

- Operating window: choose frequency between LUF and MUF on both ends and along the path; grayline minimizes D-layer losses. Equinox seasons often enhance long-path on 20 m and sometimes 40 m.