Question
How does antenna height affect the azimuthal radiation pattern of a horizontal dipole HF antenna at elevation angles higher than about 45 degrees?
Answer Options
- A) If the antenna is too high, the pattern becomes unpredictable
- B) Antenna height has no effect on the pattern
- C) If the antenna is less than 1/2 wavelength high, the azimuthal pattern is almost omnidirectional
- D) If the antenna is less than 1/2 wavelength high, radiation off the ends of the wire is eliminated
Correct Answer: C
Explanation
The ground acts as a large reflector for a horizontal antenna, and the proximity of this reflector significantly shapes the radiation pattern, particularly the high-angle lobes used for Near Vertical Incidence Skywave (NVIS).
When the antenna is less than \frac{1}{2} wavelength high, the azimuthal pattern is almost omnidirectional. The low height causes the main radiation lobe to be pointed almost straight up (high elevation angle), and the energy reflected by the ground fills in the deep nulls that would normally be off the ends of the dipole, creating near-equal radiation in all horizontal directions.
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