Near Vertical Incidence Skywave
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Near Vertical Incidence Skywave , NVIS, is a radio antenna configuration that provides usable signals in the range between groundwave and skywave distances (usually 30 to 400 miles, or 50 to 650 km). The usable frequencies are between 1.8 MHz and 15 MHz. with the most common use being between 3.5 MHz and 7.3 MHz.
NVIS configuration is a horizontally polarized (parallel with the surface of the earth) radiating element that is from 1/20th wavelength to 1/8th wavelength above the ground. That proximity to the ground forces the majority of the radiation to go straight up. Overall efficiency of the antenna can be increased by placing a ground wire slightly longer than the antenna, parallel to and directly underneath the antenna. While the ground wire is not necessary under good to excellent propagation conditions, antenna gain in the 3 dB to 6 dB range are common when the ground wire is used.
Significant increases in communication will be realized when both the transmitting station and the receiving station use NVIS configuration for their antennas.
NVIS is most useful in mountainous areas where line of sight propagation at VHF or UHF frequencies is ineffective or when the communication distance is beyond ground wave (less than 50 miles) and less than sky-wave (300 to 1500 miles). More simply stated, NVIS communication is most effective for regional use. This may be used to handle emergency communication or simply for fun.
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[edit] Reference
- ↑ Antony Wedgwood, G0TJD (April 2001). "Near Vertical Incidence Skywave". The Vintage and Military Amateur Radio Society Newsletter 16: 7–11.
