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SECTION 1 – Concept of Operation
The Model DX-Mil is a limited space transportable base antenna that operates over the 2 to 30 MHz bands to link
wideband HF communications equipment using short range (ground wave), medium range (sky wave), and long
haul RF patterns as shown in the Appendices. Rather than using only horizontal polarization, the DX-Mil utilizes
both Vertical and Horizontal polarizations to achieve uniform patterns without the need for a ground screen. To
enable the wide band operation in a confined space, the DX-Mil reduces gain to eliminate the need for a tuning
unit or coupler. Thereby, eliminating unnecessary equipment and reducing risk by reducing points of failure,
resulting in an antenna system that is rugged, reliable, and which can be erected by one person in 10 minutes.
Many HF communication systems use vertical whip antennas that are not directional. With these antennas,
communications are achieved on very short ranges by ground-wave (surface-wave propagation), and longer paths
are achieved by sky-wave propagation. An inherent characteristic of radio-wave propagation, using whip antennas,
is the zone of silence (skip zone) between the point where the ground-wave signal becomes unusable and the sky-
wave signal starts to become usable (Ref. Radio Amateur’s Handbook, Ionospheric Propagation, most editions).
Depending upon terrain, ground conductivity, operating frequency, noise levels, etc., ground-wave signals are
usable up to about 70 miles over average soil. Also, minimum distances for sky-wave paths, using whips, are
generally 200 miles (E-layer) during the day and 400 miles (F-layer) at night.
While the skip zone, described above, severely limits the usefulness of vertical antennas for short-range
communications, conditions become even worse in an adverse environment, such as a hilly or forest-type terrain.
This occurs because of the restricted range of ground-wave signals in these environments.
The inverse distance field is the field that would be present if there were no attenuation due to the surface over
which the signal is propagated. The strongest practical signals occur over seawater.
As the soil conductivity decreases or as the foliage increases, the signal strength at a distance decreases rapidly.
The important consideration for communications is not the value of signal level, but the signal-to-noise ratio.
Good ground-wave communications are expected at 25 miles at any time of the day for good ground conditions,
and the range may be as much as 100 miles for a couple of hours at midday. However, if the environment is dense
forest instead of good ground, the maximum ground-wave communication range may be 1 mile or less.
From the above discussion, it is clear that a skip zone is present when vertical antennas are used. The extent of the
skip zone is dependent upon soil conditions. For average environments, the skip zone lies between 70 and 200/300
miles; however, in extreme environments, it may include the range from 1 to 200/300 miles. The skip zone is of a
very critical range for most tactical communication systems including man-pack, vehicular, and shelter equipment.
If HF communications are to be effective in this range, different antennas and propagation modes are necessary.
The DX-Mil addresses those requirements necessitating good communications in the 0 to 300-mile range.
The solution to the short-range communication problem is the use of sky-wave instead of ground-wave
propagation on the short paths. This requires radiation from the antenna at very high elevation angles NVIS (near
vertical incidence sky-wave). This is accomplished by deploying the DX-Mil vertical element simultaneously with the
horizontally sloped elements. Radiation characteristics of the vertical element enhance DX, while radiation
characteristics of the NVIS type are achieved through the use of the NVIS element mounted above ground. Such
radiation characteristics are omni directional in azimuth and provide an l-hop range of about 300 miles. The
antenna gain varies mainly with the ground conditions below the antenna system.
