What type of signal is reflected by the ionosphere for successful amateur radio communication?

The ionosphere is a layer of the Earth's atmosphere that is ionized by solar radiation and plays a crucial role in radio wave propagation, particularly for amateur radio communication. Shortwave signals, which typically fall in the HF (high frequency) band, are able to reflect off the ionosphere due to their wavelengths being well-suited for this reflection.

These signals range from about 3 to 30 MHz and can travel long distances by bouncing off the ionosphere, enabling communication over vast areas, including international contacts. When conditions are favorable, shortwave signals can even reflect multiple times between the ground and the ionosphere, allowing for communication over thousands of miles.

In contrast, low frequency signals do not have sufficient energy to be effectively reflected by the ionosphere, instead tending to follow the curvature of the Earth. Very high frequency (VHF) and ultra high frequency (UHF) signals, while used for communication, typically have line-of-sight propagation and are not effectively reflected by the ionosphere under normal conditions. This means they are better suited for shorter distances or local communications rather than long-range, which is where shortwave signals shine due to their ability to utilize the ionosphere effectively.