Question E3C11:
What does VOACAP software model?  

Choices:  
A: AC voltage and impedance  
B: VHF radio propagation  
C: HF propagation  
D: AC current and impedance    

Correct Answer is  C: HF propagation  

Correct Answer: C — HF propagation

Why C is correct:
- VOACAP (Voice of America Coverage Analysis Program) is a prediction tool for ionospheric HF (3–30 MHz) propagation. It models skywave paths, signal-to-noise ratio, reliability, and coverage based on date/time, solar indices, antennas, power, and locations.

Why the others are incorrect:
- A: AC voltage and impedance — That’s circuit analysis, typically done with tools like SPICE, not VOACAP.
- B: VHF radio propagation — VHF (30–300 MHz) is mainly line-of-sight/tropospheric; VOACAP focuses on ionospheric HF, not VHF.
- D: AC current and impedance — Also circuit analysis, not a radio propagation model.

Useful formulas/rules of thumb for HF propagation:
- HF works when your operating frequency is between the LUF and MUF:
  $$LUF < f_{op} < MUF$$
- MUF estimate using F2-layer critical frequency:
  $$MUF \approx \frac{f_{oF2}}{\cos \theta} \quad \text{(incidence angle } \theta \text{ at the ionosphere)}$$
  Common rule of thumb for a 3000 km path:
  $$MUF_{3000} \approx 3\, f_{oF2}$$
- Practical tip: choose
  $$f_{op} \approx 0.85 \times MUF$$
  for reliable links, while staying above the LUF (which rises with D-layer absorption at lower frequencies).