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simultaneously with the horizontally sloped NVIS element. Radiation pattern characteristics of the vertical
element enhance DX, while radiation pattern characteristics of the NVIS type are achieved through the
use of the NVIS element. Such radiation characteristics are omnidirectional in azimuth and provide an l-
hop range of about 300 miles. The antenna gain varies mainly with the height of the antenna above
ground.
Because it is highly desirable to have minimum height and weight for tactical antennas, the immediate
problem becomes one of determining the minimum effective antenna height required. In order to
determine the required minimum antenna height, a minimum acceptable level of performance is
established as necessary to permit communications.
The required effective height of the antenna is found by considering the following; when a horizontal
antenna is close to ground, energy is radiated in two modes. 1) The desired NVIS mode produces
radiation with a maximum in the vertical direction. 2) The undesirable Beverage mode creates a vertical
electric field between the conductor and ground, producing vertically polarized ground-wave signal with a
maximum pattern in the direction off the NVIS wire end. Due to the proximity of the antenna to ground,
this latter mode has an efficiency that is generally poorer than a whip. To negate the undesirable mode,
the vertical element is deployed simultaneously with the NVIS element.
The shape of the radiation pattern of the horizontally sloped NVIS element is essentially constant for
heights not exceeding one-quarter wavelength. For a fixed height above ground, the amount of the input
power radiated proportionately in each of these modes is a function of the relative percentage of the
antenna input resistance characterizing each mode. Each of these, in turn, is a function of the height
above ground. The total input resistance is that portion due to the NVIS mode as the NVIS height is
varied. As the height increases, a larger part of the input signal is radiated in the NVIS mode. These
resistances are typical of these encountered over average ground.
For example, an antenna at an effective height of 0.070 wavelength is about 5 feet at 7.000 MHz. The
result of this example enables an effective height for a signal at 0.070 wavelength to be achieved by
elevating the horizontal NVIS element so that is mounted between two 5-foot supports where the NVIS
element is horizontally mounted, or by a sloping the NVIS element so that it is connected at between 5-7
feet on the top bolt of the Alpha Match and sloped down to the ground in the same manner that a guy wire
would be deployed. An advantage of the sloping NVIS configuration is the vertically polarized component,
which produces desired affects at low frequencies and also permits compatibility with the vertical element
to enable DX propagation.