NVIS Army Field Manual 24-18 Appx M

Outline:

A Heterogeneous Layer-Based Trustworthiness Model for Long ...

- NEAR-VERTICAL INCIDENCE SKY-WAVE (NVIS) PROPAGATION CONCEPT
M-1. Evaluation of Communications Techniques
M-2. Problems Encountered in Propagation of Radio Waves
M-3. Concept of Near-Vertical Incidence Sky-Wave Radiation
M-4. Assessment of Characteristics of Common Antennas
M-5. Orientation of Antenna Wire
M-6. Problems in Using the NVIS Concept
M-7. Advantages in Using the NVIS Concept
M-8. Conditions Under Which to Use the NVIS Concept

M-1. Evaluation of Communications Techniques

The standard communications techniques used in the past will not support the widely deployed and the fast-moving formations we intend to use to counter the modern threat. Coupling this with the problems that can be expected in deploying multichannel LOS systems with relays to keep up with present and future operation, high frequency (HF) radio and the near-vertical incidence sky-wave (NVIS) mode take on new importance. High frequency radio is quickly deployable, securable, and capable of data transmission. It will be the first, and frequently the only, means of communicating with fast-moving or widely separated units. It may also provide the first long-range system to recover from a nuclear attack. With this reliance on HF radio, communications planners, commanders, and operators must be familiar with NVIS techniques and their applications and shortcomings in order to provide more reliable communications.

M-2. Problems Encountered in Propagation of Radio Waves

Under ideal conditions, the ground wave component of a radio wave becomes unusable at about 80 kilometers (50 mi). However, under actual field conditions, this range can be much less, sometimes as little as 3 kilometers (2 mi). Sky waves, generated by standard antennas, will not return to earth at a range of less than 161 kilometers (100 mi), leaving a skip zone where HF communications will not function. This skip zone can prevent communication with units such as long-range patrols, armored cavalry, air defense early warning teams, and many division stations. NVIS mode solves this problem by reflecting energy back to earth at all angles, ensuring skip-zone-free communications.

M-3. Concept of Near-Vertical Incidence Sky-Wave Radiation

The NVIS mode, also known as the near-vertical incidence sky-wave, allows energy radiated in a near-vertical direction to be reflected back to earth in an omnidirectional pattern without skip zones. This mode relies on properly selecting the frequency and angle of incident signal upon the ionosphere. By radiating energy on a low enough frequency and at angles greater than 75 or 80 degrees from the horizontal, the ionospheric layers reflect the energy in an umbrella-type pattern. This mode ensures skip-zone-free communications and reduces the impact of terrain and vegetation interference.

M-4. Assessment of Characteristics of Common Antennas

To obtain the required radiation characteristics for short-range HF communications, half-wave dipole antennas located from one-quarter to one-tenth wavelength above the ground can be used. Antennas such as the half-wave Shirley folded dipole or the fan dipole perform well in the NVIS mode. For mobile operations, vehicular-mounted antennas like the standard 5-meter whip can be used. The selection and orientation of these antennas are essential to minimize ground-wave radiation and maximize energy directed towards the zenith.

M-5. Orientation of Antenna Wire

While in traditional long-haul paths, dipole antennas are oriented with the broadside pointing towards the receiving station(s), this is not necessary in the NVIS mode. For NVIS operation, antenna orientation does not matter since all the energy is directed upward and returns to earth in an omnidirectional pattern. It allows the dipole to be erected at any convenient orientation without affecting the communication with other stations. However, when operating near the magnetic dip equator, the dipole antennas should be oriented in a magnetically north-south direction to increase receive signal levels.

M-6. Problems in Using the NVIS Concept

While the NVIS technique provides skip-zone-free communications, there are a few drawbacks that must be understood. The interference between ground wave and sky wave can cause destructive interference, but proper antenna selection can minimize this effect. High take-off angles require careful selection and placement of antennas to maximize radiation towards the zenith. Critical frequency selection is important as radiated energy above the critical frequency will not be reflected by the ionosphere. This restricts the NVIS mode to specific frequency ranges based on the path length. Despite these drawbacks, NVIS mode offers advantages such as constant received signal levels, low probability of geolocation, and successful use of low-power HF sets.

M-7. Advantages in Using the NVIS Concept

After overcoming the problems associated with the NVIS concept, several advantages come with its use. In the tactical environment, skip-zone-free omnidirectional communications allow for constant received signal levels without being affected by terrain. The NVIS mode is harder to jam due to ground-wave jammers being subject to path loss, resulting in a lower probability of geolocation. Additionally, low-power HF sets can be used successfully in the NVIS mode, reducing the probabilities of intercept/detection. These advantages make the NVIS concept a valuable tool in military communications.

M-8. Conditions Under Which to Use the NVIS Concept

The NVIS concept should be considered under the following conditions: when the area of operations does not support ground-wave HF communications, when tactical deployment places stations in anticipated skip zones, when operating in heavy wet jungle or areas with high signal attenuation, when prominent terrain features are not under friendly control, and when operating against enemy ground-wave jammers and direction finders. By utilizing the NVIS mode strategically, better communication outcomes can be achieved in challenging environments.

In conclusion, the NVIS Army Field Manual 24-18 Appendix M provides valuable insights into the near-vertical incidence sky-wave propagation concept. By understanding the evaluation, problems, concepts, assessment, orientation, problems, advantages, and conditions associated with the NVIS mode, communication planners, commanders, and operators can achieve more reliable communications particularly in fast-moving or widely separated units. This manual equips individuals with the necessary knowledge to navigate the challenges in deploying multichannel LOS systems and underscores the importance of high frequency radio in modern military operations.