6-79. HF signals travel much farther by sky-wave propagation than by ground-wave propagation. Sky-wave propagation is the bending (refraction) of the radio signal by a region of the atmosphere called the ionosphere.
6-80. The ionosphere is an electrically charged (ionized) region of the atmosphere that extends from an altitude of about 60 to 1,000 km (37 to 620 miles) above the earth’s surface. Energy from the sun ionizes the atmosphere in this altitudinal range, and the electrical charge there refracts (bends) some radio signal that enters it, sending the signal back to the earth.
6-81. The area that affects HF communications the most lies between the altitudes of 48 km (29.6 miles, which lies below or inside the ionosphere) to 500 km (310 miles). This 440 km (273 mile) area is divided into four incremental altitudinal ranges: D, E, F1, and F2 (Table 6-12 and Figure 6-8).
Table 6-12. High frequency ranges in ionosphere.
D -- 48 to 88 km (30 to 55 miles) NOTE:
E -- 88 to 136 km (55 to 85 miles) F1 -- 136 to 248 km (88 to 155 miles)
F2 -- 248 to 400 km (155 to 250 miles)
↔
F -- 136 to 400 km (88 to 250 miles)Figure 6-8. Structure of ionosphere.
6-82. The majority of HF sky-wave communications depend on the F1 and F2 regions. The F2 region is used the most for long-range daytime communications.
6-83. The E region is the next lower region. It is present 24 hours a day, although at night it is much weaker. The E region is the first region with enough charge to bend radio signals. At times, parts of the E-region become highly charged. This can either help or block HF communications. These highly charged areas are called "sporadic E." They occur most often during the summer.
6-84. The D-region is closest to earth and only exists during the day. It cannot bend a radio signal back to earth, but it does play an important role in HF communication. The D-region absorbs energy from the radio signal passing through it, thereby reducing the strength of the signal.
6-85. The bending of the radio signal by the ionosphere depends on the frequency of the radio signal, the degree of ionization in the ionosphere, and the angle at which the radio signal strikes the ionosphere. At a vertical (straight up) angle, the highest frequency bent back to earth is called the critical frequency. Each region of the ionosphere (E, F1, and F2) has a separate critical frequency. For a vertical angle, signals above the highest critical frequency pass through all ionospheric regions and into outer space. Frequencies below the critical frequency of a region are bent back to the earth by that region; however, if the frequency is too low, the signal is absorbed by the D region. To have HF sky-wave communication, a radio signal must be a high enough frequency to pass through the D region, but not so high a frequency that it passes
through the refracting region. Thus, radio operators must have current propagation charts from which to choose the most effective frequency during a given time period. To achieve an NVIS effect, the radio operator subtracts 20 percent from frequencies propagated on commercial computer propagation programs. 6-86. The angle at which a radio signal strikes the ionosphere plays an important part in sky-wave communication. As previously stated, any frequency above the critical frequency passes through the refracting region. If the radio signal having a frequency above the critical frequency is sent at an angle, the signal is bent back to earth instead of passing through the region. This can be compared to skipping stones across a pond. If a stone is thrown straight down at the water, it penetrates the surface. If a stone is thrown at an angle to the pond, the stone skips across the pond. For every circuit, there is an optimum angle above the horizon called the takeoff angle. This angle produces the strongest signal at the receiving station. This optimum takeoff angle is used to select the antenna for a specific circuit. By placing a dipole antenna between one-eighth and one-quarter wavelength above ground level, the radio operator achieves an NVIS effect, and he reduces or eliminates any skip zone (Figure 6-9).
Figure 6-9. HF skip zone and distance.
6-87. Depending on the frequency, antenna, and other factors, an area may exist between the longest ground-wave range and the shortest sky-wave range where no signal exists. This is called the skip zone and no communication is possible. The NVIS effect can eliminate this problem.
6-88. Multiple frequencies are usually needed to maintain sky-wave communication. As a minimum, two frequencies, one for day and one for night are normally required.
CLASSIFICATION
6-89. Antennas are classified by the directions in which they can radiate energy. The three classifications include omnidirectional antennas (all directions), bidirectional antennas (two directions), or directional (one direction). A directional antenna is the best choice--if it works--because its signal is the most difficult for the enemy to locate.
D
IRECTIONAL6-90. This antenna's single lobe of energy sends a unidirectional signal (Figure 6-10). The width of the signal ranges from a narrow pencil beam to a 60-degree arc, depending on the type of directional antenna chosen.
Figure 6-10. Unidirectional antenna pattern.
Application
6-91. Directional antennas are used on long-range, point-to-point circuits that need concentrated radio energy to ensure a reliable signal.
Orientation
6-92. A directional antenna concentrates most of its energy in one direction, so it requires careful orientation.
Detection
6-93. The enemy has a hard time determining the origin of directional antennas, which minimizes interference.
Adaptation of Bidirectional Antennas for Directional Use
6-94. Adding a terminating resistor to absorb the energy of the second lobe allows directional use of a bidirectional (long-wire or sloping "V") antenna. The terminating resistor must match the antenna. That is, it must be able to absorb one-half of the power output of the connected transmitter and provide 400 to 600 ohms of resistance.
B
IDIRECTIONAL6-95. A bidirectional antenna (Figure 6-11) has two opposite lobes of radio energy, with an area of null energy (no energy) between them. The lobes produce two strong signals in opposite directions, and weaker ones in all other directions.
Figure 6-11. Bidirectional antenna pattern.
Application
6-96. Bidirectional antennas are usually used on point-to-point circuits and in situations where the antenna null can be positioned to reduce or block signals that could interfere with reception.
Orientation
6-97. To work properly (radiate in the desired directions), a bidirectional antenna must be oriented to the ground wave, and this is difficult to do. Lowering the antenna to create a near-vertical-incidence skywave (NVIS) effect makes this more difficult, because it increases the radiation pattern. A bidirectional antenna is best used near other antennas, which should be placed in its null to reduce interference and interaction between the antennas.
Examples
6-98. The bidirectional antennas most commonly used in tactical situations are the sloping-"V," random-length wire, and half-wave dipole.