ARRL Technician Practice Exam

Which part of the atmosphere enables the propagation of radio signals around the world?

1. Troposphere

2. Stratosphere

3. The ionosphere

4. Exosphere

The correct answer is: The ionosphere

The ionosphere plays a critical role in the propagation of radio signals around the world. This region of the Earth's atmosphere is located approximately 30 to 1,000 miles above the surface and is characterized by a high concentration of ionized particles. When radio waves travel from the Earth's surface, they can encounter these ionized layers in the ionosphere. The ionization occurs mainly due to solar radiation and results in the reflection or refraction of radio signals back towards the Earth, allowing for long-distance communication. This property of the ionosphere is particularly important for high-frequency (HF) radio communications, which rely on skywave propagation. In contrast, other atmospheric layers such as the troposphere, stratosphere, and exosphere do not have the same ionization characteristics conducive for reflecting radio waves over long distances. The troposphere is where weather phenomena occur and primarily affects VHF and UHF communications rather than long-distance propagation. The stratosphere, located above the troposphere, is relatively stable and does not contribute to the reflecting properties of radio signals. The exosphere, being the outermost layer, transitions into outer space and lacks the density needed for effective radio wave propagation. Thus, the ionosphere remains the key layer that