Ham Radio General Class Practice Test

How does the feed-point impedance of a 1/2 wave dipole antenna change as the antenna is lowered from 1/4 wave above ground?

1. It steadily increases

2. It steadily decreases

3. It peaks at about 1/8 wavelength above ground

4. It is unaffected by the height above ground

The correct answer is: It steadily decreases

The feed-point impedance of a 1/2 wave dipole antenna is influenced by its height above ground due to the proximity of the ground and the surrounding environment. When a 1/2 wave dipole is installed at a height of 1/4 wavelength above ground, the feed-point impedance is relatively high, typically around 70-75 ohms. As the antenna is lowered towards the ground, the impedance tends to decrease steadily. This decrease in impedance occurs because the ground affects the radiation pattern and the antenna's ability to effectively radiate radio waves. As the antenna gets closer to the ground, it encounters more ground losses and interactions with the ground plane, which contribute to a lower feed-point impedance. The pattern continues until the dipole is significantly closer to the ground, where the impedance response can become more complicated due to multiple factors, including ground conductivity and nearby objects. Understanding this change in impedance is crucial for operators wanting to match their dipole antenna's feed-point impedance with lines and equipment for effective communication, particularly when adjusting height for different bands or environmental conditions.