Near vertical incidence skywave (NVIS) is an HF propagation mode in which signals are transmitted towards, and returned from, the ionosphere almost vertically in order to provide local or regional coverage. NVIS is also often used in environments that are challenging for traditional HF propagation modes, such as in mountainous regions. This educational note is divided into two sections. The first section provides a brief overview of HF and the more common HF propagation modes. NVIS can be considered a special case of skywave propagation, and thus emphasis is placed on ionospheric propagation, in particular the role of frequency and incidence angle. The second section discusses the technical principles underlying NVIS, the different types of antennas commonly used for NVIS operation, as well as the technical and operational aspects of NVIS.
