A thorough and fully annotated discussion of Near Vertical Incidence Skywave (NVIS) is available in the research paper, “Radio Communication via Near Vertical Incidence Skywave Propagation: An Overview,” by Ben A. Witvliet, PE5B/5R8DS, and Rosa Ma Alsina-Pagès.
First investigated in the 1920s, NVIS propagation was rediscovered during World War II as “an essential means to establish communications in large war zones such as the D-Day invasion in Normandy,” the paper notes, adding that the US Army subsequently sponsored a lot of NVIS field research, especially between 1966 and 1973. More recently, NVIS has become a popular means to enable close-in communication on Amateur Radio HF bands between 3 and 10 MHZ. NVIS can be used for radio communication in a large area (200-kilometer radius) without any intermediate manmade infrastructure, and it has been found to be especially suited for disaster relief communication, among other applications, according to the paper.
“A comprehensive overview of NVIS research is given, covering propagation, antennas, diversity, modulation, and coding,” the Abstract explains. “Both the bigger picture and the important details are given, as well as the relation between them.” As the paper describes it, in NVIS propagation, electromagnetic waves are sent nearly vertically toward the ionosphere, and, with appropriate frequency selection, these waves are reflected back to Earth.
“The great reflection height of 80 to 350 kilometers results in a large footprint and homogeneous field strength across that footprint,” the paper says. “Due to the steep radiation angles large objects such as mountain slopes or high buildings cannot block the radio path.”
As for NVIS antennas, the paper stipulates that important parameters are antenna diagram, polarization, and bandwidth. “As only high elevation angles contribute to NVIS propagation, optimizing the antenna diagram for these elevation angles will increase the effectively transmitted power and improve the signal-to-interference ratio at reception.”