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miles, preventing communications with the desired station. The other term called ground wave is
where your signal does reach someone closer. A ground wave signal can go up to approximately 50
miles if conditions, including terrain and obstacles, are favorable.
Using no skip zone or ground wave, the NVIS mode is used for making reliable HF
communications below 10 MHz
effective for a range to 600 miles. The
NVIS propagation mode works best on
HF below 10 MHz since these high
angle radio waves are reflected back to
Earth. Most often a low dipole is the
best antenna to use and will provide
reliable communications. This makes a
NVIS antenna an excellent choice for
emergency communications and for
staying in contact with other amateur
radio operators in nearby locations.
Dipoles only exhibit directionality once they reach 1/2 wavelengths above ground. However, NVIS
antennas are located from 1/4 to 1/10 wavelength above ground.
Vertical RF energy radiated at a low enough frequency is reflected back to earth at all angles.
The effect is similar to taking your garden hose with a fog nozzle and pointing it straight upwards.
The water coming back down gives you an omni-directional pattern without dead spots. It's a
continuous circular radiation pattern coming back down. Since it is omni-directional, dipole
orientation is not important.
The higher the angle, the lower the frequency
needed to work properly. Therefore lower
amateur radio frequencies such as 40 and 80
meters are ideal for NVIS use. NVIS
generally requires takeoff angles of 70 degrees
or higher. Radio waves directed vertically at
frequencies higher than the critical frequency
pass through the ionized layer out into space.
Typical expected ranges are up to 300 miles
when using NVIS antennas which make them
ideal for close in communications (not for
DXing).
The DX Engineering versions of the NVIS antenna have been designed and tested in real-world
situations and found to offer great performance for NVIS communications on both 80 and 40
meters.
