High frequency
The dominant means of long-distance communication in this band is skywave ("skip") propagation, in which radio waves directed at an angle into the sky refract back to Earth from layers of ionized atoms in the ionosphere.
[4] By this method HF radio waves can travel beyond the horizon, around the curve of the Earth, and can be received at intercontinental distances.
This means that at longer distances, where the waves graze the ionosphere at a very blunt angle, the MUF may be much higher.
At worst, when a band is "dead", no communication beyond the limited groundwave paths is possible no matter what powers, antennas or other technologies are brought to bear.
When a transcontinental or worldwide path is open on a particular frequency, digital, SSB and Morse code communication is possible using surprisingly low transmission powers, often of the order of milliwatts, provided suitable antennas are in use at both ends and that there is little or no artificial or natural interference.
The main uses of the high frequency spectrum are: The high frequency band is very popular with amateur radio operators, who can take advantage of direct, long-distance (often inter-continental) communications and the "thrill factor" resulting from making contacts in variable conditions.
International shortwave broadcasting utilizes this set of frequencies, as well as a seemingly declining number of "utility" users (marine, aviation, military, and diplomatic interests), who have, in recent years, been swayed over to less volatile means of communication (for example, via satellites), but may maintain HF stations after switch-over for back-up purposes.
Other standards development such as STANAG 5066 provides for error free data communications through the use of ARQ protocols.
Noise, especially man-made interference from electronic devices, tends to have a great effect on the HF bands.
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.
These systems incorporate frequencies down to 2 MHz to include the 2182 kHz international distress and calling channel.
