This article discusses selection of HF frequencies for regional communications using information available to the public on the Internet. The reader is encouraged to review the many other sources of information available on the Internet and in hard copy which exist on the subject of HF propagation. This is intended to be a brief, basic, non-technical presentation of the writer's understanding based upon research, discussion and 34 years' amateur radio experience on HF frequencies, much of which has included HF regional nets. Helpful references will be annotated at the end of this article. The essential element of HF radio communication is the fact that, under certain conditions, radio signals will be refracted (or "reflected") back to Earth by the ionosphere. The ionosphere is that region of the upper atmosphere, ranging between about 50 and 200 miles above the surface of the Earth, where rarified gases are affected by the sun's radiation in such a way that the reflection of radio waves can occur. HF radio waves can also be received over relatively short distances without the involvement of the ionosphere. This type of propagation is called "ground wave." However, the useful range of groundwave propagation is so short that it is not of much use for region-wide communications and will not be discussed further in this article.

The key advantage of HF radio communications for region-wide coverage is that it can provide uniform coverage of the entire area within about 300 miles and up to 1,000 miles, regardless of terrain, when the atmosphere and geomagnetic fields are suitable and when the appropriate frequency is used. The disadvantages are that solar eruptions can make HF completely unusable for this purpose and, of course, the physical size of the antennas needed.

Selection of an appropriate frequency is key. Success can also be improved by selection of an antenna which will maximize radiation in the vertical direction, commonly referred to as "near vertical incidence skywave" (NVIS) radiation. A complete discussion of antenna design is beyond the scope of this article. However, many useful antenna designs and accompanying analyses are available at L. B. Cebik's website, and others.

Selection of a net frequency should be based upon the following factors:

• The size of the region to be covered. For the purposes of this article, a "region" is considered to be at least 50 miles and up to 1,000 miles wide.
• The frequencies, or bands of frequencies, available to the radio operators involved which is limited by their licensing; and
• The state of the ionosphere, in particular, the "critical frequency," at the time of the net. The critical frequency, abbreviated "foF2," is the highest frequency at which radios waves directed straight up will be reflected by the ionosphere back to Earth.

Just as a pane of glass, or the surface of a lake will reflect light more efficiently if the light shines across it at a low angle, the ionosphere will reflect radio signals more efficiently at lower angles. For that reason, the ionosphere will reflect radio waves at frequencies higher than the critical frequency, if those waves approach the ionosphere at lower angle. The highest frequency at which a radio wave will reach a distant station is called the "maximum usable frequency" (MUF) for that distance. In general, the greater the distance, the lower the angle of radiation which will reach that distance, and the higher the MUF. Thus, the critical frequency and the MUF are similar concepts. The difference is that the MUF is related to a significant distance from the transmitter and the critical frequency is related to a signal directed straight up from the transmitter -- or "distance zero."

The situation created when a radio signal is on a frequency which is above the critical frequency but still lower than the MUF for a distant station is commonly observed on HF radio frequencies. In this situation, the more distant station hears the signal, but stations closer to the transmitter do not hear it because the high angle signals which would reach the closer stations are not reflected by the ionosphere. Those closer stations are said to be in the "skip zone" of the transmitter. That is why two stations perhaps 100 miles apart can each hear a station 2,000 miles away, but cannot hear each other.

The same phenomenon is observed occurring gradually on evening nets. On a 40 meter net established shortly after sunset, all stations might hear each other. As the Sun's affect on the ionosphere fades, each station gradually begins to hear more distant stations better than locals until, eventually, the local stations are no longer audible. Some amateur radio operators refer to this effect as "the band going long." It is, actually, the effect of the critical frequency dropping below the net's operating frequency because of the loss of solar radiation. (There may be other changes in the atmosphere which also contribute to this effect, such as the appearance or disappearance of lower layers of the ionosphere which effectively lowers or raises the apparent height of the reflective layer).

"Rule of thumb:" The optimum frequency for a regional net will be approximately 85% of the "critical frequency." Frequencies close to this calculated one will be reflected, even at high angles of incidence, by the ionosphere. The result will be that high angle signals will return to Earth like the water droplets from a fire hose pointed straight up. The transmitted signal will "rain down" around the transmitter site and thoroughly cover the area around the transmitter out to several hundred miles. Another analogy is a flashlight directed at the center of the ceiling in a darkened room. Light is diffused and reflected by the ceiling and illuminates the entire room. Also, because the radio waves are returned to Earth at high angles, they reach into valleys and canyons. That is how near vertical incidence skywave accomplishes region-wide coverage.

Frequencies higher than the critical frequency will pass through the ionosphere into outer space, if they are directed "straight up." Radio frequencies lower than 85% of the critical frequency will be reflected back toward Earth. However, frequencies which are significantly lower that foF2 may be diminished by absorption at lower levels of the atmosphere, resulting in poor reception -- a topic for another day.

The following examples apply the above principles to real-world scenarios:

• A net is intended to cover an area within 300 miles of the net control. The critical frequency is known to be 6.1 Mhz. The net control station (NCS) is considering a choice between a 7 Mhz frequency in the 40 meter band and a 3.9 Mhz frequency in the 75 meter band. Doing a quick analysis, it seems that 85% of 6.1 Mhz is approximately 5.2 Mhz. Since the 3.9 Mhz frequency is below the critical frequency, the NCS selects that frequency for the net operation. This frequency should allow all stations within the range of the net to hear each other and the NCS. If the 40 meter frequency were used, there may be some stations close to the NCS who could not hear that station.
• The same net is scheduled, but the critical frequency is 8.2 Mhz. The NCS determines that 85% of 8.5 Mhz is about 7.2 Mhz. Under these conditions, the 40 meter frequency may be the best choice since it would allow continuous coverage from 0-300 miles from the NCS. In fact, it would probably provide propagation even farther.

http://digisonde.haystack.edu/latestFrames.htm

http://www.ips.gov.au/Main.php?CatID=6&SecID=4&SecName=North%20America&SubSecID=3&SubSecName=Ionospheric%20Map

By referring to these sources, informed choices can be made when selecting the operating frequencies for regional HF radio nets. Further readings on this subject are available at many sources, including the following:

http://www.qsl.net/wb5ude/nvis/

http://www.tactical-link.com/nvis_discussion_page.htm

http://www.raynet-hf.net/HFNVIS.html