text: Communications
  Figure 6-7. Components of ground wave.
  Reflected Wave
  6-77. This is a wave that bounces off the earth on its way to the receiving antenna.
  Space Wave
  6-78. This wave is the combination of a reflected wave and a direct wave.
  SKY-WAVE PROPAGATION
  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 and F2 combine into F at night:
  F1 -- 136 to 248 km (88 to 155 miles)
  ↔
  F -- 136 to 400 km (88 to 250 miles)
  F2 -- 248 to 400 km (155 to 250 miles)