What is the longest planning range of the Near Vertical Incidence Skywave (NVIS) antenna?

The Near Vertical Incidence Skywave (NVIS) antenna is designed to communicate over relatively short distances, typically within a range that can be effectively used for local or regional communications. The longest planning range for NVIS antennas is generally accepted to be around 300 miles under optimal conditions.

This range is mainly due to the way NVIS propagates radio waves. It relies on the ionosphere to reflect signals back to the Earth, and this method is most effective for communication distances of 0 to about 600 miles, with the maximum effective distance being closer to the 300-mile mark, depending on several factors such as frequency, atmospheric conditions, and the angle of incidence.

In contrast, the other options pertain to distances that are either shorter or beyond the practical range typically achieved with NVIS configurations. While NVIS can sometimes reach distances close to 400 miles or more under exceptional conditions, 300 miles is the distance that most practitioners would consider to be the upper limit of a realistic planning range for reliable communication with this type of antenna. Therefore, the selection of 300 miles accurately reflects the typical maximum planning range for NVIS antennas.