NVIS

NVIS (Near vertical incidence skywave)

NVIS is a method of using skywave radio propagation for medium range distances, usually up to about 400 miles.  The angle of radiation pattern from the antenna is deliberately high, causing the signal to be transmitted upwards and reflected down at a steep angle, making the skip distance short and can therefore be used to circumvent line-of-sight barriers.  The most effective frequencies for NVIS are between 1.8 MHz and 8 MHz.  Above 8 MHz, the probability of success begins to decrease, dropping to near zero at 30 MHz as the frequency means that the RF is no longer reflected by the ionosphere.  Experiments show the NVIS antennas to work best when approximately a 1/4  wavelength above ground, parallel to the earth and when a ground wire slightly longer than the radiating element is placed on the ground parallel to it.