Page 5
SECTION 1 – Concept of Operation
The tuner free & directional Alpha Multiband Antenna is a multipurpose antenna system,
which can be configured to launch your signal at your target.
Many short-range HF communication circuits 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 whip 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 whip
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. Most tactical requirements necessitate good communications in the
0 to 300-mile range. If HF communications are to be effective in this range, different
antennas and propagation modes are necessary.
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 MilStick vertical element simultaneously with the horizontally sloped NVIS
element. 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
