We usually think of HF as a long range communications solution which is impacted by sun spot activity, time of day and weather. To ensure somewhat reliable or consistent communications, beam antennas and amplifiers are required and even then there are no guarantees, some locations may hear but others will not. For shorter range communications we think of VHF/UHF mobile radios and repeaters. Depending on the repeater location, stations 100 km apart can communicate while mobile.
But what about situations where there is no repeater, repeaters have failed or the existing repeaters just don't link the two locations that need to communicate.
Near Vertical Incident Skywave (NVIS) has been around for a long time (since W.W.II) and has applications in military and commercial communications. Some civilian emergency management organizations also use it and there are a few amateurs across North America that use and experiment with NVIS. So what is NVIS?
"Near vertical incident skywave means forcing your radio signals to travel straight up (i.e., 80-90 degrees) and back down. This achieves radio coverage in circle having a radius of 300 miles and more. Stop and think about that for a moment. Complete coverage within such a circle on frequencies between 2 and 10 or 12 Megahertz. Some readers may wonder what's so good about this. We are talking about dependable local area high frequency communications -- the type we need for tactical public safety communications in the Radio Amateur Civil Emergency Service. In tactical communications we don't want DX."
Copied from www.sedata.net/nvis.html, an excellent introduction to NVIS.
"NVIS , short for Near Vertical Incident Skywave, utilizes high-angle skywave paths between stations instead of ground wave or surface wave in order to communicate via HF radio. NVIS was originally evaluated by U.S. Army Forces in Thailand during the Vietnam conflict in the mid-1960's It was found that Mobile stations, using whip antennas bent parallel to the ground, could communicate more reliably with their base-stations. Signal strengths would be weaker using high-angle skywave but communications would be more reliable, less subject to fading, and consistent between stations. This was because the intervening terrain was less of an absorber of signals. Terrain obstructions between stations, such as hills, mountainous areas, jungle growth, built-up areas with tall buildings, no longer become path obstructions with stations when NVIS techniques are employed. For distances out to 400 miles between stations, one F-layer hop, at vertical angles of 45 degrees or higher are used. It is not necessary to have high power transmitters. Typical 100 watt power levels are fine. It is necessary that all stations on an NVIS radio network use antennas that are parallel to the ground and the frequencies used are chosen via a radio propagation prediction program in order to have best results."
Copied from a background on NVIS By Patricia Gibbons WA6UBE: http://www.sanjoseca.gov/oes/races/hfradio.htm
NVIS operation is best in the 2 MHz to 10 MHz range, which makes 80m and 40m prime frequencies, with 30m having some possibility. With the new 5 MHz (60m) allocation in the US and UK, this could provide another useful frequency if the allocation were available in Canada. The importance of having a range of frequencies, is to ensure consistent communications regardless of time of day or other conditions. The operating frequency must change during the day and night periods.