HF Propagation Modes

HF (3—30 MHz) radio waves can travel in a variety of modes between two ground-based transceivers. Skywaves are the most useful because they provide the longest range possible. The higher the reflecting region, the greater the range for a given angle of incidence.

1. Line of Sight (LOS) propagation occurs when stations are directly visible to each other.
2. Ground wave is propagation of surface wave (vertical polarization only) effective over a conductive ground, below 2 MHz.
3. Skywave is a form of skip propagation caused by reflections from the ionosphere,, typically effective for frequencies ranging from 3 to 30 MHz.
  ^* In late spring or early fall, low VHF (30 to 150 MHz) signals can be unpredictably "reflected" back to Earth via Sporadic E.
  ^**A special mode of Skywave is NVIS: a near vertical ״reflection״ by the ionosphere, in the lower HF bands 1.8-10 MHz.

Usually, Line of Sight signals would propagate directly between transmitter and receiver, but there could be complex reflections (by conductive surfaces) ^{See for example: ElectronicsNotes, Kogelnik, Wiki}.

Ground wave: Surface propagation occurs when signals follow the contour of the Earth, over hills and beyond the horizon. It is most effective over salty sea water or good conductive ground (vertical polarization only). The effective distance of ground waves drops significantly above 2 Mhz.

Skywave is the mode of propagation in which the radio signal is "reflected" by the ionosphere and returns to Earth. It is most effective at frequencies beween 3 and 30 MHz and can be used for long-range communication. HF radio waves are generally refracted by the ionospheric regions E, F1, and F2, and are only rarely reflected by Aurora and/or Meteor Scatter. There are sometimes ducting effects as illustrated here.

Read more about Modes of HF Propagation the main page. See also an index of terms for HF Radio Propagarion.