text: Chapter 6
  POLARIZATION
  6-70. Polarization is the relationship of radio energy radiated by an antenna to the earth. The most
  common polarizations are horizontal (parallel to the earth’s surface) and vertical (perpendicular to the
  earth’s surface). Others, such as circular and elliptical, also exist. A vertical antenna normally radiates a
  vertically polarized signal, and vice versa.
  GROUND WAVES
  6-71. For best communication with HF ground waves, both the sending and receiving antennas should
  have the same polarization. Vertical polarization works best for HF ground-wave propagation.
  SKY WAVES
  6-72. For HF sky-wave propagation, the sending and receiving antennas need not have the same
  polarization, because the ionosphere will bend the waves, thus randomly changing their polarization
  anyway. However, horizontal polarization works best for HF sky-wave propagation.
  RADIO WAVE PROPAGATION
  6-73. HF communications can be established using either ground- or sky-wave propagation. With
  low-powered, man-pack radios, ground-wave communication can be established out to about 30 km,
  depending on conditions. High-powered, vehicle-mounted equipment allows communication out to about
  100 km. Sky-wave communications range from several to thousands of kilometers.
  GROUND-WAVE PROPAGATION
  6-74. Ground-wave propagation means sending a radio signal along or near the surface of the earth. The
  ground-wave signal has three parts: the direct, reflected, and surface waves (Figure 6-7).
  Surface Wave
  6-75. The surface wave travels, as the name implies, along the surface of the earth. It is the usual means
  of ground-wave communication. The surface wave depends on the type of surface that lies between the two
  antennas. With a good conducting surface, such as seawater, long ground-wave distances are possible.
  Poor surfaces, such as sand or frozen ground, shorten the distance the surface wave can travel. Heavy
  vegetation or mountainous terrain can do the same.
  Direct Wave
  6-76. The direct wave travels from one antenna to the other in what is called the line-of-sight mode.
  Maximum line-of-sight distance depends on the height of the antenna above ground. The higher the
  antenna, the longer the LOS. Because radio signals travel in the air, any obstruction between the antennas,
  such as a mountain, can block or reduce the signal. For an antenna 10 feet above the ground, the maximum
  LOS is 8 km (5 miles).