Question E3A06:
What should be done to continue a long-distance contact when the MUF for that path decreases due to darkness?  

Choices:  
A: Switch to a higher frequency HF band  
B: Switch to a lower frequency HF band  
C: Change to an antenna with a higher takeoff angle  
D: Change to an antenna with greater beam width    

Correct Answer is  B: Switch to a lower frequency HF band  

To continue a long-distance contact as darkness lowers the MUF, you move down in frequency so your signal remains below the new MUF and can still refract via the F layer.

Why B is correct (Switch to a lower frequency HF band):
- At night, ionization in the F layer drops, reducing the critical frequency and thus the MUF.  
- If your operating frequency stays above the new MUF, the wave penetrates the ionosphere and the path dies. Dropping to a lower HF band keeps you below MUF so the path continues.

Why the other choices are incorrect:
- A: Switch to a higher frequency HF band  
  Going higher takes you even farther above the falling MUF, so the signal is even less likely to refract—path will fail.

- C: Change to an antenna with a higher takeoff angle  
  Long-distance (DX) paths generally use low takeoff angles. A higher takeoff angle shortens hop distance and does not reliably compensate for a MUF drop on the same long path. The practical, reliable fix is to lower frequency.

- D: Change to an antenna with greater beam width  
  Beam width affects directivity, not whether the ionosphere will refract your signal. It does not address the MUF limitation.

Useful formulas and rules of thumb:
- Secant law for oblique incidence:
  $$\text{MUF} \approx f_{oF2} \cdot \sec i$$
  where:
  - $f_{oF2}$ = F2-layer critical frequency (vertical incidence), which drops after sunset
  - $i$ = angle of incidence relative to the vertical at the layer

- Optimum working frequency (OWF) for reliability:
  $$\text{OWF} \approx 0.85 \times \text{MUF}$$

Rule of thumb:
- As night falls, $f_{oF2}$ and MUF decrease, so shift down in band (e.g., 20 m to 30/40 m). D-layer absorption also decreases at night, lowering the LUF, which helps lower HF bands work better.