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NVIS Army FM 24-18[backcolor=rgba(255, 255, 255, 0.901961)]Appendix M with GraphicsNEAR-VERTICAL INCIDENCE SKY-WAVE (NVIS) PROPAGATION 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, ground wave component of a radio wave becomes unusable at about 80 kilometers (50 mi) (fig 2-12). Under actual field conditions, this range can be much less, sometimes as little as 3 kilometers (2 mi). Sky waves, generated by standard antennas (for example, doublets) which efficiently launch the sky wave, will not return to earth at a range of less than 161 kilometers (100 mi). This can leave a skip zone of at least 80 to 113 kilometers (50 to 70 mi) where HF communications will not function. This means that units such as long-range patrols, armored cavalry deployed as advance or covering forces, air defense early warning teams, and many division-corps, division-brigade, division-DISCOM and division-DIVARTY stations are in the skip zone and thus unreachable by HF radio even though HF is a primary means of communication to these units. M-3. Concept of Near-Vertical Incidence Sky-Wave Radiation Energy radiated in a near-vertical incidence direction is not reflected down to a pinpoint on the Earth’s surface. If it is radiated on too high a frequency, the energy penetrates the ionosphere and continues on out into space. Energy radiated on a low enough frequency is reflected back to earth at all angles (including the zenith), resulting in the energy striking the earth in an omnidirectional pattern without dead spots (that is, without a skip zone). Such a mode is called a near-vertical incidence sky wave (NVIS). The concept is illustrated in figure M-1. This effect is similar to taking a hose with a fog nozzle and pointing it straight up. The water falling back to earth covers a circular pattern continuously out to a given distance. A typical receive signal pattern for antenna AS-2259/GR is shown in figure M-2, and the path length and incident angle are shown in figure M-3. A typical elevation plane pattern is shown in figure M-4. The main difference between this short-range NVIS mode and the standard long-range sky-wave HF mode is the lower frequency required to avoid penetrating the ionosphere and the angle of incident signal upon the ionosphere. In order to attain a NVIS effect, the energy must be radiated strong enough at angles greater than about 75 or 80 degrees from the horizontal on a frequency that the ionosphere will reflect at that location and time. The ionospheric layers will reflect this energy in an umbrella-type pattern with no skip zone. Any ground wave present with the NVIS signal will result in undesirable wave interference effects (such as, fading) if the amplitudes are comparable. However, proper antenna selection will reduce ground-wave radiated energy to a minimum, and this will reduce the fading problems. Ranges for the NVIS mode are shown in figure M-3 for typical ionosphere height and take-off angles. Since NVIS paths are purely sky wave, the path losses are nearly constant at about 110 dB +10 dB. Relative gain performance of the AS-2259/GR NVIS antenna is shown in figure M-5. This is significant for the tactical communicator because all the energy arriving at the receiving antenna is coming from above at about the same strength over all of the communications ranges of interest. This means the effect of terrain and vegetation (when operating from defiladed positions such as valleys) are greatly reduced, and the receive signal strength will not vary greatly.
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