Ham Radio General Class Practice Test

What kind of antenna exhibits the lowest radiation resistance?

1. A long dipole antenna

2. A short dipole antenna

3. A Yagi-Uda antenna

4. A vertical antenna

The correct answer is: A short dipole antenna

A short dipole antenna exhibits the lowest radiation resistance among the options provided. Radiation resistance is a measure of how effectively an antenna converts input power into radio waves. A full-sized dipole antenna, typically around half a wavelength long, has a radiation resistance of approximately 73 ohms. However, when the dipole is shortened, as in the case of a short dipole antenna, its radiation resistance decreases significantly, often dropping to around 20 ohms or even lower, depending on how much shorter it is relative to its resonance. In general, shorter antennas tend to have lower radiation resistance because they are less efficient at radiating energy. The current distribution along the length of the antenna is not as uniform, leading to reduced radiation efficiency. Other antennas listed, such as a long dipole, Yagi-Uda, and vertical antennas, tend to have higher radiation resistances because they are designed to be more efficient in converting electrical power into electromagnetic waves. Thus, the characteristic of short dipole antenna makes it the correct answer when identifying which antenna exhibits the lowest radiation resistance.