GeneralG9B07
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?
B
Answer
Antennas and feed lines
Type
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
Answer Notes
The feed point impedance of a half-wave dipole in free space is approximately 73 ohms. However, when the antenna is brought close to the earth (especially below 1/4 wavelength), the ground begins to act like a mirror, creating electrical reflections that interact with the antenna.
As the height is reduced toward 1/10 wavelength, the capacitive coupling with the ground increases, which causes the radiation resistance and the overall feed point impedance to steadily decrease. At very low heights, the impedance can drop to 10 or 20 ohms.
Options suggesting the impedance increases or remains unaffected ignore the strong near-field interaction between the antenna and the conductive earth.
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How does the feed point impedance of a 1/2 wave dipole change as the feed point is moved from the center toward the ends?