Amateur Radio Emergency Service

Amateur Radio Emergency Service
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NVIS Antenna Systems

. . . Definition
Near-Vertical Incident Skywave ("NVIS") is a theory of radio propagation using F-Layer atmospheric refraction around 65° to 90° enabling low-power local and region communications within a radius of 300 to 400 miles.


. . . System Concept
NVIS should be viewed as a system, in the sense that stations which are similarly equipped will be able better to communicate within the range of the system. Stations outside the range of F-Layer refraction, which would be emphasized by a low elevation pattern, are de-emphasized with the high-elevation pattern NVIS system. The "system" consist of the NVIS antennas (all with high-elevation patterns), the operator's knowledge, skill, and experience.


. . . History
NVIS antenna systems were pioneered by the Germans in WWII, and were known as "rail" or "cage" antenna.. NVIS systems were widely used by the US forces in Vietnam. NVIS is now being studied, promoted, and deployed by ARES and other emergency communicators for use in terrain where line of sight V/UHF communications is not possible.


. . . Range
NVIS systems have a reliable range within a radius of 300 to 400 miles using low power (5-100 watts) transmitters.


. . . Power
NVIS systems should be limited to 100 Watts, because more power frequently causes increased groundwave, resulting in phase-distorted reception issues.


. . . Antennas
NVIS antennas are usually low wires or loaded whips, mounted horizontally, less than 1/8th W in height. A typical NVIS system will include two dipoles (80-Meters @ 121 feet & 40-Meters @ 65 feet) mounted at right angles about ten feet above the ground.


. . . Users' Group
< http://groups.yahoo.com/group/nvis/ >

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