NVIS HF Antennas
I became interested recently with the concept of NVIS (Near Vertical Incidence Skywave) Antennas. The NVIS antenna is ideal for local and emergency communications on HF.
Portablity and their unique performance envelope make them worth looking at for any amateur serious about reliable HF communications. Probably the biggest users of NVIS antennas are the military and FEMA.
What is NVIS?
"NVIS, or Near Vertical Incidence Skywave, refers to a radio propagation mode which involves the use of antennas with a very high radiation angle, approaching or reaching 90 degrees (straight up), along with selection of an appropriate frequency below the critical frequency, to establish reliable communications over a radius of 0-300 miles or so, give or take 100 miles. Although not all radio amateurs have heard the term NVIS, many have used that mode when making nearby contacts on 160 meters or 80 meters at night, or 80 meters or 40 meters during the day. They may have thought of these nearby contacts as necessarily involving the use of groundwave propagation, but many such contacts involve no groundwave signal at all, or, if the groundwave signal is involved, it may hinder, instead of help. Deliberate exploitation of NVIS is best achieved using antenna installations which achieve some balance between minimizing groundwave (low takeoff angle) radiation, and maximizing near vertical incidence skywave (very high takeoff angle) radiation."
The paragraph above is an excerpt from an excellent source of technical information on NVIS principles which can be found here thanks to James Glover, WB5UDE. Another special thank you is due Dr. Carl Jelinek,N6VNG whose NVIS antenna construction article written in 1998 is the basis for this project.
The most famous of the NVIS military antennas... recently in use during the War in Iraq by US Forces... is the AS-2259 NVIS antenna, manufactured originally by Collins Radio (Model 637-K1) and Telex (Model 1990) - Telex recently sold it's antenna line to Associated Industries in North Hollywood,CA and the fate of the Model 1990 is unknown at this time. Harris Communications manufactures the antenna as the (Model RF-1936). The most interesting thing about the AS-2259 is that the hollow 1.25" tube that makes up it's mast also serves as a low loss feed line. Here is the Army Technical Manual for the AS-2259/GR Antenna.
In the pages that follow I'll walk you through construction of a "homebrew" version of the famous AS-2259 !