High frequency (HF) radio frequencies are between 3 and 30 MHz. Also known as the decameter band or decameter wave as the wavelengths range from one to ten decameters (ten to one hundred metres). Frequencies immediately below HF are denoted Medium-frequency (MF), and the next higher frequencies are known as Very high frequency (VHF). Shortwave (2.310 - 25.820 MHz) overlaps and is slightly lower than HF.

Propagation characteristics

Since the ionosphere often refracts HF radio waves quite well (a phenomenon known as skywave propagation), this range is extensively used for medium and long range radio communication. However, suitability of this portion of the spectrum for such communication varies greatly with a complex combination of factors:

• Sunlight/darkness at site of transmission and reception
• Transmitter/receiver proximity to terminator
• Season
• Sunspot cycle
• Solar activity
• Polar aurora

These and other factors contribute, at each point in time for a given communication path, to a

• Maximum usable frequency (MUF)
• Lowest usable high frequency (LUF) and a
• Frequency of optimum transmission (FOT)

Exploitation of, and limits imposed by, these characteristics

When all factors are at their optimum, worldwide communication is possible on HF. At many other times it is possible to make contact across and between continents or oceans. At worst, when a band is 'dead', no communication beyond the limited groundwave paths is possible no matter what powers, antennae or other technologies are brought to bear. When a transcontinental or worldwide path is open on a particular frequency, digital, SSB and CW communication is possible using surprisingly low transmission powers, often of the order of tens of watts, provided suitable antennae are in use at both ends and that there is little or no man-made or natural interference. On such an open band, interference originating over a wide area affects many potential users. These issues are significant to military, safety and amateur radio users of the HF bands.

Uses

CB radios operate in the higher portion of the range (around 27 MHz), as do some studio-to-transmitter (STL) radio links. Some modes of communication, such as continuous wave morse code transmissions (especially by amateur radio operators) and single sideband voice transmissions are more common in the HF range than on other frequencies, because of their bandwidth-conserving nature, but broadband modes, such as TV transmissions, are generally prohibited by HF's relatively small chunk of electromagnetic spectrum space.

Noise, especially man-made interference from electronic devices, tends to have a great effect on the HF bands. In recent years, concerns have risen among certain users of the HF spectrum over "broadband over power lines" (BPL) Internet access, which is believed to have an almost destructive effect on HF communications. This is due to the frequencies on which BPL operates (typically corresponding with the HF band) and the tendency for the BPL "signal" to leak from power lines. Some BPL providers have installed "notch filters" to block out certain portions of the spectrum (namely the amateur radio bands), but a great amount of controversy over the deployment of this access method remains.