This summary is based upon the assumption that the reader has some familiarity with the subject of Near Vertical Incidence Skywave HF-SSB propagation for communications between 1.8 and around 10 Megahertz. NVIS is essential to anyone requiring reliable HF communication from one to 400 miles. Such users include the RACES, Operation SECURE, the Civil Air Patrol, FEMA, the U.S. Forest Service, MARS and others. This information is not found in conventional technical publications and (least of all) in the field of Amateur Radio. Having said that, here again are the highlights of NVIS (pronounced "niviss").
For practical communications plans and operations, NVIS functions between 1.8 and around 10 Megahertz. Much above that and the signals penetrate the iononosphereic layer instead of the desired reflection back to earth.
Using a "NVIS antenna", NVIS provides total coverage for a radius of 300 to 400 miles from any such station.
A NVIS antenna is always horizontal. A vertical antenna can never be used, including mobiles.
A NVIS antenna has omnidirectional radiation; in other words, it makes no difference how you orient your antenna.
A NVIS antenna is low; it MUST be low. Attempt to keep it no more than twenty feet above electrical or earth ground.
A multi-frequency NVIS antenna requires a remote, and automatic, antenna tuner at the end of the coaxial cable and before the antenna system.
If you use a dipole antenna with an automatic antenna tuner to operate on more than one frequency, cut the dipole to the lowest frequency to be used with the conventional formula.
An existing dipole antenna over twenty feet high can be expediently modified to obtain a degree of NVIS performance. This is done by allowing the feedpoint to sag ten to fifteen feet below the ends of the antenna. A horizontal broadband antenna, such as a B and W, may be used without an automatic antenna for a base station.
End-fed long wire antennas are NOT recommended. Unbalanced antennas are prone to creating interference to telephones and other electronic systems in the vicinity.
When the user has a choice of several frequencies, the best choice is generally ten percent below the MUF or Maximum Usable Frequency obtained from propagation programs.
Now, and for the next several years, the solar activity is and will affect HF communicators in a manner most are not accustomed. Amateur Radio operators will find that 80 meters will often work better in the daytime than 40 meter and that 160 will be better than 80 at night. The Civil Air Patrol will find that it should use 2374 kilohertz at night instead of 4585 for more reliable communications. Remember, we are talking about communications necessary up to about 400 miles away. This may mean some necessary equipment and antenna changes. One thing is certain -- this condition will be with us for several years and justify the expense.
In summation, HF-SSB and NVIS signals will climb out of the deepest canyons, hop the highest mountains, never require any repeaters or other intermediate relays. And some thought High Frequency radio was an ancient art!
Using a "NVIS antenna", NVIS provides total coverage for a radius of 300 to 400 miles from any such station.
A NVIS antenna is always horizontal. A vertical antenna can never be used, including mobiles.
A NVIS antenna has omnidirectional radiation; in other words, it makes no difference how you orient your antenna.
A NVIS antenna is low; it MUST be low. Attempt to keep it no more than twenty feet above electrical or earth ground.
A multi-frequency NVIS antenna requires a remote, and automatic, antenna tuner at the end of the coaxial cable and before the antenna system.
If you use a dipole antenna with an automatic antenna tuner to operate on more than one frequency, cut the dipole to the lowest frequency to be used with the conventional formula.
An existing dipole antenna over twenty feet high can be expediently modified to obtain a degree of NVIS performance. This is done by allowing the feedpoint to sag ten to fifteen feet below the ends of the antenna. A horizontal broadband antenna, such as a B and W, may be used without an automatic antenna for a base station.
End-fed long wire antennas are NOT recommended. Unbalanced antennas are prone to creating interference to telephones and other electronic systems in the vicinity.
When the user has a choice of several frequencies, the best choice is generally ten percent below the MUF or Maximum Usable Frequency obtained from propagation programs.
Now, and for the next several years, the solar activity is and will affect HF communicators in a manner most are not accustomed. Amateur Radio operators will find that 80 meters will often work better in the daytime than 40 meter and that 160 will be better than 80 at night. The Civil Air Patrol will find that it should use 2374 kilohertz at night instead of 4585 for more reliable communications. Remember, we are talking about communications necessary up to about 400 miles away. This may mean some necessary equipment and antenna changes. One thing is certain -- this condition will be with us for several years and justify the expense.
In summation, HF-SSB and NVIS signals will climb out of the deepest canyons, hop the highest mountains, never require any repeaters or other intermediate relays. And some thought High Frequency radio was an ancient art!
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