Question E3B13:
What type of polarization is supported by ground-wave propagation?  

Choices:  
A: Vertical  
B: Horizontal  
C: Circular  
D: Elliptical    

Correct Answer is  A: Vertical  

Correct answer: A — Vertical

Why vertical is correct:
- Ground-wave (surface-wave) propagation travels along the Earth’s surface. A true surface wave is supported only when the electric field is vertical to the ground.
- At the Earth’s surface, the tangential electric field must be very small. With vertical polarization, the electric field is mostly normal (perpendicular) to the surface, minimizing lossy surface currents and allowing a Zenneck/Norton surface wave to “hug” the ground.
- Practical proof: MF/LF and AM broadcast stations use vertical antennas to maximize ground-wave coverage.

Why the others are incorrect:
- Horizontal: The electric field is tangential to the ground, driving strong surface currents and causing rapid attenuation. Horizontal components are largely “shorted out” by the ground, so a horizontal ground wave does not propagate effectively.
- Circular and Elliptical: These are combinations of horizontal and vertical components. The horizontal component is quickly attenuated by the ground, so the wave does not maintain circular/elliptical polarization as a ground wave. Any surviving field tends toward vertical but with significant loss—thus not “supported” as a ground wave polarization.

Useful rules of thumb:
- Use vertical polarization for ground-wave coverage.
- Ground-wave range improves with better ground conductivity (best over seawater) and degrades with poorer conductivity (dry/rocky terrain).
- Ground-wave attenuation increases with frequency; it is most useful at LF/MF and can be marginal at lower HF. A rough guideline: below a few MHz, ground wave can be practical; above that, it diminishes rapidly.
- Any arbitrary polarization can be decomposed into vertical and horizontal components; near the ground, only the vertical component effectively supports a surface wave.