AdvancedA-007-007-006
How does antenna height affect the horizontal (azimuthal) radiation pattern of a horizontal dipole HF antenna?
C
Answer
Interference, EMC, and safety
Type
A
If the antenna is too high, the pattern becomes unpredictable
B
If the antenna is less than one-half wavelength high, radiation off the ends of the wire is eliminated
C
If the antenna is less than one-half wavelength high, reflected radio waves from the ground significantly distort the pattern
D
Antenna height has no effect on the pattern
Answer Notes
A horizontal dipole in free space has a classic figure-eight radiation pattern. However, placing it near the ground drastically alters this horizontal (azimuthal) pattern because ground reflections interact with the direct waves.
When the antenna is mounted less than one-half wavelength high, these ground reflections significantly distort the horizontal pattern. It becomes somewhat omnidirectional and pushes the bulk of the radiation straight upward. It is only at heights of one-half wavelength or higher that the antenna begins to clearly exhibit its traditional broadside directivity.
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When a half-wave dipole antenna is installed one-half wavelength above ground, the:
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For long distance propagation, the vertical radiation angle of the energy from the antenna should be: