Near vertical incidence skywave

It is used for military and paramilitary communications, broadcasting,[1] especially in the tropics, and by radio amateurs for nearby contacts circumventing line-of-sight barriers.

The radio waves travel near-vertically upwards into the ionosphere, where they are refracted back down and can be received within a circular region up to 650 km (400 miles) from the transmitter.

There is no fundamental difference between NVIS and conventional skywave propagation; the practical distinction arises solely from different desirable radiation patterns of the antennas (near vertical for NVIS, near horizontal for conventional long-range skywave propagation).

Usable frequencies are dictated by local ionospheric conditions, which have a strong systematic dependence on geographical location.

Optimum NVIS frequencies tend to be higher towards the tropics and lower towards the arctic regions.

NVIS is most useful in mountainous areas where line-of-sight propagation is ineffective, or when the communication distance is beyond the 80 km (50 miles) range of groundwave (or the terrain is so rugged and barren that groundwave is not effective), and less than the 500–2,400 km (300–1,500 miles) range of lower-angle sky-wave propagation.

An NVIS antenna configuration is a horizontally polarized (parallel with the surface of the earth) radiating element that is from ⁠1/ 20 ⁠th wavelength (λ) to ⁠ 1 /4⁠ wave above the ground.

[6] That proximity to the ground forces the majority of the radiation to go straight up, causing NVIS propagation to occur.

[8] Ground wires are more necessary when using lower dipoles over poor soils as without them considerable energy goes into heat and not the radio waves.