NVIS is short for Near Vertical Incidence Skywave. This is a useful communications method which allows for high frequency communications to be used by reflecting signals off the D layer of the atmosphere.
The goal of NVIS is to permit communications only within a 400 mile radius or less. Often times, high frequency communications in the lower end of the 1.8 to 30 MHz range is designed to reach much further distances.
Since reliable VHF communications is generally limited to line of sight, being able to set up NVIS communications is very important when other systems may fail and a last option is needed. Mechanisms such as repeaters, internet or satellite communication are man made and may be unavailable at times.
NVIS is not to be confused with ground wave communications which could accomplish the same results, but would require much more transmitter power. NVIS communication can be accomplished with as little as a few watts by comparison.
Another benefit of NVIS is it can be used to communicate over naturally occurring land features such as mountains.
Being able to setup equipment for NVIS communications is important and is not very complicated.
How can you experiment with NVIS?
High frequency communications antenna are often thought to be most efficient at one quarter wavelength above ground or higher. Instead, NVIS actually makes use of the ground as part of the antenna system by breaking this rule.
Simply taking a dipole antenna for the appropriate frequency or other types of antenna and keeping them lower to the ground allows the signals to be reflected off the ground and then nearly sent straight up before before reflected back down in the 400 mile radius we are looking for.
There is a great explanation and actual use cases explained in this video from March of 2021 thanks to two of the more inspiring radio amateur organizations in our region.
If you are looking to try NVIS communications, it is easy to do this on your own, at home in your backyard or even with friends such as those involved in some form of auxiliary communications group.