Question
How does the feed point impedance of a horizontal 1/2 wave dipole antenna change as the antenna height is reduced to 1/10 wavelength above ground?
Answer Options
- A) It steadily increases
- B) It steadily decreases
- C) It peaks at about 1/8 wavelength above ground
- D) It is unaffected by the height above ground
Correct Answer: B
Explanation
The ground acts as a reflector, and the antenna’s feed point impedance is determined by the combination of direct radiation and the reflected wave from the ground. As a horizontal dipole is lowered from \frac{1}{2} wavelength to very close to the ground, the phase relationship between the direct wave and the reflected wave changes.
Specifically, the feed point impedance of a horizontal \frac{1}{2} wave dipole antenna steadily decreases as its height is reduced toward the ground. At very low heights (e.g., 1/10 wavelength), the impedance can drop to 10 \text{ ohms} or less, requiring a special matching network.
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