High Frequency (HF) Radio

HF radio waves occupy the band between 3 and 30 megahertz (MHz), and are part of what is known as the shortwave band. HF transmit using “skywave” or “skip” propagation, giving HF the ability to send and receive over long distances. HF radio waves occupy a spectrum that interacts with the earth’s atmosphere in a very specific way – when broadcast at an angle towards the they will refract off the ionosphere and back towards the earth’s surface where it will bounce back multiple times. HF radio wave are capable of broadcasting signals beyond the horizon and around the curvature of the earth’s surface. In optimal conditions and using the appropriate set up, HF waves can even be transmitted between continents, however this should never be relied on as a primary mode of intercontinental communication. HF radio waves refracting off the ionosphere greatly reduce dead spots and radio “shadows” cast by hills our mountains, however dense surrounding buildings may still effect HF usage.

While HF may offer an advantage in the distance of its communication, it also comes with limitations. Notably, the equipment required to transmit and receive HF signals is bulky and large, and requires a significantly larger antenna and a larger energy source. Generally speaking, there are no good solutions for handheld mobile HF radios used – HF is almost always limited to vehicles and stationary buildings.

Because HF uses the ionosphere, it is affected by the sun. The time of day affects the transmission and reception of HF radio waves. Typically, lower-level HF waves work best at night, while higher-level HF waves work best during the day. The sun’s sunspot cycle also has an impact. The more ionizing radiation emitted by the sun, the greater the ionosphere’s density, hence the more substantial bending effects of HF waves.