What is near vertical incidence skywave (NVIS) propagation?

Near Vertical Incidence Skywave (NVIS) is a specific propagation technique used to provide reliable, continuous coverage over short distances, typically between 0 and 400 miles. It works by using an antenna configuration that radiates the signal almost straight up at a very steep (high) elevation angle. Because the signal hits the ionosphere from directly below, it reflects straight back down like water bouncing off a ceiling, completely filling in what would otherwise be a skip zone. NVIS relies on lower frequency bands (usually 160, 80, or 40 meters) that can reflect back at steep angles without passing completely through the ionosphere into space. It is distinctly different from standard long-distance skip, which relies on low elevation angles to bounce signals as far along the horizon as possible.