text: Communications
  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.
  DIRECTIONAL
  6-90. 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.