Ham Radio General Class Practice Test

What effect do radio waves usually experience when they exceed the MUF?

1. They penetrate the ionosphere and are lost in space

2. They are reflected off the ionosphere and return to the Earth

3. They enter a state of resonance

4. They are transmitted without interference

The correct answer is: They penetrate the ionosphere and are lost in space

When radio waves exceed the Maximum Usable Frequency (MUF) for a given path and time, they typically penetrate the ionosphere instead of being reflected back to Earth. The MUF is the highest frequency at which radio waves can be transmitted via skywave propagation, meaning they can reflect off the ionosphere and travel long distances. Frequencies above the MUF do not have the necessary critical angle to be reflected; hence, they pass through the ionosphere into space, resulting in a loss of the transmitted signal. The phenomenon is tied to the properties of the ionosphere, which can vary in density depending on factors such as solar activity and time of day. When the frequency of the radio wave is too high, it simply continues into space, leading to the loss of signal for radio communications meant to rely on ionospheric reflection. This characteristic is essential for operators to understand in order to select appropriate frequencies for their communications based on current ionospheric conditions.