Near Vertical Incidence Skywave

The NVIS concept is easy to grasp. It makes use of the well understood properties of low-band HF signal propagation - their ability to reflect off of the ionosphere with relatively little signal loss. Amateur Radio operators normally leverage this reflectivity for long distance (DX) contacts by radiating their signals at an angle so they skip across the ionosphere to stations hundreds or thousands of miles away.

But what if, in an emergency situation, our close-range line of sight communications infrastructure is down? Think a 2017 Hurricane Maria scenario. What happens when Amateur Radio, EMS and public service point-to-point and repeater infrastructure is not working and we need to establish communications with the next town, the next county or the next state?

Using your regular Amateur Radio HF transceiver and a horizontal antenna resonant on 40 or 80 meters, you can easily re-establish close-in communications with other stations. The idea is to simply set your antenna close to the ground (less than 1/4 wavelength for the frequency you are using) so that the Earth acts as a signal reflector, pushing your signals near vertically into the atmosphere. The ionosphere acts like a mirror, sending those signals back to earth at the same reflected angle. So if a signal from your NVIS antenna hits the ionosphere at 0 degrees of angle (straight up), it will be reflected back to Earth at 0 degrees (straight down).

But this is an imperfect world. Our reflective surfaces (the Earth and the ionosphere) are not perfectly reflective, and our antenna setups are never optimal. Because of this we know that only a small part of our signal will hit the ionosphere at 0 degrees. Most of the signal will hit the ionosphere at angles close to zero, and be reflected back at the same angles. This is the beauty of NVIS (remember, the NV in NVIS stands for Near Vertical). Your signal is 'rained down' over a relatively close area, even reaching hard to contact stations that are well shielded by terrain or urban canyons.