Question E3B05:
Which of the following paths is most likely to support long-distance propagation on 160 meters?  

Choices:  
A: A path entirely in sunlight  
B: Paths at high latitudes  
C: A direct north-south path  
D: A path entirely in darkness    

Correct Answer is  D: A path entirely in darkness  

Correct answer: D — A path entirely in darkness

Why D is correct
- On 160 meters (1.8 MHz), the main limiter to long-distance ionospheric propagation is D-layer absorption, which is strongest in sunlight.
- At night, the D layer largely disappears, so absorption drops dramatically and signals can refract from the F layer for long-haul contacts.
- Thus, a path entirely in darkness maximizes the chance for DX on 160 m.

Why the other choices are incorrect
- A: A path entirely in sunlight
  - Daylight creates a strong D layer, causing heavy absorption at low frequencies. On 160 m, this often drives the lowest usable frequency above 1.8 MHz, preventing ionospheric skip.
- B: Paths at high latitudes
  - High-latitude and auroral-zone paths suffer additional absorption and disturbances (auroral/geomagnetic absorption), which are particularly harmful at MF/LF like 160 m.
- C: A direct north-south path
  - There’s no inherent north-south advantage on 160 m. In fact, N–S great-circle routes from mid-latitudes often approach auroral regions, increasing absorption. The key is darkness, not path orientation.

Useful formulas and rules of thumb
- D-layer absorption increases rapidly at low frequency:
  - $A \propto \dfrac{1}{f^2}$
  - Lower $f$ → much higher absorption in daylight.
- For ionospheric propagation you want:
  - $f_{\text{op}} > \text{LUF}$ and $f_{\text{op}} < \text{MUF}$
  - On 160 m, $f_{\text{op}} \approx 1.8\ \text{MHz}$ is usually well below MUF; the controlling factor is the LUF, which drops at night when D-layer absorption vanishes.
- Practical rules:
  - Best 160 m DX: full darkness along the entire path, especially in winter and during low geomagnetic activity.
  - Avoid paths crossing auroral zones; grayline can help but full darkness is usually better for long-haul on 160 m.