Which antenna system and operating frequency are most suitable for Near Vertical Incidence (NVIS) communications?

Near Vertical Incidence Skywave (NVIS) propagation is used for regional communications, typically within a 300 to 500 km radius. To achieve this, the antenna must radiate its signal almost straight up (at a very high angle) so it reflects directly back down to the surrounding area. A horizontal dipole placed close to the ground (less than 1/4 wavelength high) produces a radiation pattern that is mostly directed straight upwards, making it ideal for NVIS. In contrast, vertical antennas have a null in the straight-up direction and radiate at low angles, making them entirely unsuited for NVIS. Furthermore, the operating frequency must be below the ionosphere's current critical frequency. If a signal is sent straight up at a frequency above the critical frequency, it will penetrate the ionosphere and escape into space instead of being reflected back to Earth.