The RECON provides a multipurpose antenna system, which can be configured to launch your signal in the manner as circumstances require. Many short-range HF communication circuits use vertical whip antennas. With these antennas, communications are achieved on very short ranges by ground-wave (surfacewave 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 is 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 manpack, 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 Page 4 accomplished by deploying the RECON as depicted on Pages 14 & 15. Radiation characteristics of the NVIS type are achieved through the use of horizontal antennas mounted above ground up to a height of about one-quarter wavelength. 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 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.
The desired NVIS mode produces radiation with a maximum in the vertical direction. The undesirable Beverage mode creates a vertical electric field between the conductor and ground, producing vertically polarized ground-wave signal with a maximum in the direction off the EMCOMM wire ends. Due to the proximity of the antenna to ground, this latter mode has an efficiency that is generally poorer than a whip. The shape of the radiation pattern of the horizontal ECOMM 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 ECOMM mode as the ECOMM height is varied. As the height increases, a larger part of the input signal is radiated in the ECOMM mode. These resistances are typical of these encountered over average ground.
For example, an antenna at an effective height of 0.035 wavelength is about 10 feet at 3.5 MHz. The result of this example enables an effective height for a signal at 0.035 wavelength to be achieved by elevating the horizontal ECOMM element so that is mounted between two 10-foot supports or by a sloping the ECOMM element so that it is mounted from a taller single mast at its feed-point. An advantage of the sloping ECOMM configuration is the vertically polarized component, which produces desired affects at low frequencies and also permits compatibility with whip antennas where propagation conditions permit.