Ham Radio General Class Practice Test

What usually happens to radio waves with frequencies below the Lowest Usable Frequency (LUF)?

1. They are bent back to the Earth

2. They pass through the ionosphere

3. They are completely absorbed by the ionosphere

4. They are bent and trapped in the ionosphere to circle the Earth

The correct answer is: They are completely absorbed by the ionosphere

Radio waves with frequencies below the Lowest Usable Frequency (LUF) are typically absorbed by the ionosphere, which is a layer of the Earth's atmosphere ionized by solar radiation. This absorption occurs because lower frequencies do not possess enough energy to reflect off the ionosphere effectively, causing a significant portion of this energy to be dissipated or absorbed by the ionized particles present in that layer. As frequencies decrease and fall below the LUF, the ability of the ionosphere to reflect these signals diminishes greatly, leading to a loss of signal rather than allowing any transmission or reflection back to the Earth. When radio waves are unable to reflect, they often fail to propagate over long distances, which is crucial for successful communication in amateur radio. This phenomenon is particularly relevant for low-frequency signals, as it limits their usability. Frequencies that are too low simply cannot be utilized for effective communication, thus rendering them ineffective for amateur radio operators or any type of long-distance radio communication.