High frequency

Lore & Background

The dominant means of long-distance communication in the HF band is skywave propagation, where radio waves directed at an angle into the sky refract back to Earth from layers of ionized atoms in the ionosphere. This allows signals to travel beyond the horizon and around the curve of the Earth, reaching intercontinental distances. However, suitability varies greatly with factors such as sunlight or darkness at transmission and reception sites, proximity to the solar terminator, season, sunspot cycle, solar activity, and polar aurora. The frequencies at which communication is possible are specified by the maximum usable frequency (MUF), lowest usable high frequency (LUF), and frequency of optimum transmission (FOT). The MUF regularly drops below 10 MHz in darkness during winter, while in summer daylight it can surpass 30 MHz. The lowest usable frequency depends on absorption in the D-layer of the ionosphere, which is stronger at low frequencies and with increased solar activity; total absorption often occurs below 5 MHz during daytime. As a result, the usable spectrum shifts toward lower frequencies and into the MF range during winter nights, while higher frequencies tend to be more usable in summer, often into the lower VHF range.

Reader's Guide

The HF band is used for military and governmental communication, aviation air-to-ground links, amateur radio, international shortwave broadcasting, maritime sea-to-shore and ship-to-ship services, over-the-horizon radar, the Global Maritime Distress and Safety System (GMDSS), citizen's band radio (generally 26–28 MHz), and coastal ocean dynamics applications radar. Amateur radio operators value HF for direct, long-distance communications and the variable propagation conditions. International shortwave broadcasting also uses these frequencies, though some utility users (marine, aviation, military, diplomatic) have shifted to satellite communication, often retaining HF as backup. The development of Automatic Link Establishment technology (MIL-STD-188-141) and higher-speed modems (MIL-STD-188-110C, supporting data rates up to 120 kilobit/s) has revived HF use in government networks. Standards such as STANAG 5066 provide error-free data communications via ARQ protocols. Continuous wave Morse code and single sideband voice are common in HF due to their bandwidth-conserving nature, while broadband modes like TV are generally prohibited by HF's limited spectrum. Noise, especially man-made interference from electronic devices, greatly affects HF bands; broadband over power lines (BPL) has raised concerns due to its destructive effect on HF communications. In aviation, HF systems are required for all trans-oceanic flights, incorporating frequencies down to 2 MHz to include the 2182 kHz international distress and calling channel. The upper section of HF (26.5–30 MHz) shares characteristics with lower VHF and is used for local communications including CB radios, studio-to-transmitter links, radio control for models, and paging transmitters. Some RFID tags also utilize HF.

Did You Know?

• The maximum usable frequency in HF can drop below 10 MHz during winter nights and surpass 30 MHz in summer daylight.
• When a transcontinental path is open on HF, communication is possible using transmission powers as low as milliwatts.
• The development of MIL-STD-188-110C modems supports data rates up to 120 kilobit/s on HF.