Near Vertical Incidence Skywave (NVIS)
Posted by swgraham2
I love DX. I love chatting to interesting people all over the world and making new friends. For short range chatting I use our Monongalia Wireless Association W8MWA Repeater on the 144/440 frequencies. Sometimes it’s more difficult to talk short distances than it is to talk half way around the world. Let’s consider Near Vertical Incidence Skywave (NVIS) antennas with our HF radios.
NVIS is something every Ham needs to learn about. The antenna system is not difficult to construct and can serve an important function for shorter range communications, especially in time of an emergency when normal local / regional communication systems are down for some reason. (i.e. cell towers, cellphones, landlines, etc.) Hams may be called into service quickly.
Here is what Wikipedia says about NVIS communications…
Near vertical incidence skywave, or NVIS, is a skywave radio-wave propagation path that provides usable signals in the range between groundwave and conventional skywave distances—usually 30–400 miles (50–650 km). It is used for military and paramilitarycommunications, broadcasting, especially in the tropics, and by radio amateurs. The radio waves travel near-vertically upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km from the transmitter. If the frequency is too high (that is, above the critical frequency of the ionospheric F layer), refraction fails to occur and if it is too low, absorption in the ionospheric D layer may reduce the signal strength.
The most reliable frequencies for NVIS communications are between 1.8 MHz and 8 MHz. Above 8 MHz, the probability of success begins to decrease, dropping to near zero at 30 MHz. Usable frequencies are dictated by local ionospheric conditions, which have a strong systematic dependence on geographical location. Common bands used in amateur radio at mid-latitudes are 3.5 MHz at night and 7 MHz during daylight, with experimental use of 5 MHz (60-meter) frequencies. Broadcasting uses the tropical broadcast bands between 2.3 and 5.06 MHz, and the international broadcast bands between 3.9 and 6.2 MHz, Military NVIS communications mostly take place on 2-4 MHz at night and on 5-7 MHz during daylight.
Optimum NVIS frequencies tend to be higher towards the tropics and lower towards the arctic regions. They are also higher during high sunspot activity years. The usable frequencies change from day to night, because sunlight causes the lowest layer of the ionosphere, called the D layer, to increase, causing attenuation of low frequencies during the day  while the maximum usable frequency (MUF) which is the critical frequency of the F layer rises with greater sunlight.
NVIS is most useful in mountainous areas where line-of-sight propagation at VHF or UHF frequencies is ineffective or when the communication distance is beyond the 50-mile (80 km) range of groundwave, and less than the 300–1500-mile (500–2500 km) range of lower angle sky-wave. Another interesting aspect of NVIS communication is, that direction finding of the sender is more difficult than for ground-wave communication (i.e. VHF or UHF). For broadcasters, NVIS allows coverage of an entire medium-sized country at much lower cost than with VHF (FM), and daytime coverage similar to MW (AM) nighttime coverage at lower cost and often with less interference. https://en.wikipedia.org/wiki/Near_vertical_incidence_skywave
Below are a few very good links to articles for the nuts & bolts of putting together a simple and good NVIS antenna.
Here is a sample video by NG9D with an 80 Meter End Fed NVIS Field Antenna.
About swgraham2Mountain Mists is an opportunity to develop some thought processes individually as I journey through this wonderful world; as someone who values highly effective management techniques and the art of leading teams; and through my association with West Virginia University as a professional technologist and network manager specializing in digital signage.
Posted on February 4, 2016, in Amateur Radio, Antennas, CW, Digital Modes, Electronic Theory, Emergency Preparedness, Field Day, Ham Radio, Ham Radio Articles, Ham Videos, HF Band, Homebrewing & Kits, Projects, Propagation, Radio, Radio Operations, Receiver, SSB, Station Accessories, Transmitter, Uncategorized and tagged 80 Meters, 80 Meters Ham Band, Amateur Radio, antenna, emergency communication, Emergency Preparedness, Ham Radio, Homebrewing, KB8FIR, Monongalia Wireless Association, NG9D, NVIS, nvis antenna, Operating Practices, prepper, Spence Graham, Spencer Graham, Spencer W. Graham, Spencer W. Graham II, W8MWA, Wire, WT8WV, WVUSWG. Bookmark the permalink. Leave a comment.