5.2 HF Propagation

This section is intended only to give general understanding of HF propagation, which is very complex. Its characteristics vary with time of day, time of year, location, and conditions (such as sunspot activity) that impact the ionosphere. An excellent article by Richard Groller in the June 1990 issue of the Journal of Electronic Defense ("Single Station Location HF Direction Finding") is suggested as a starting point for further study. The next suggestion is a handbook such as Reference Data for Radio Engineers ( RDRE), which includes typical curves for HF propagation. Finally, for specific ionospheric conditions, propagation parameters, and other examples, the Federal Communication Commission has a Web site with loads of data (http://www.fcc.gov).

In this section, we will discuss the ionosphere, ionospheric reflection, HF propagation paths, and single-site locator operation. The primary references for this section are Mr. Groller's article and the RDRE.

HF propagation can be line of sight, ground wave, or sky wave. Where line of sight exists, propagation is predicted by the formulas presented for VHF and UHF propagation in Section 5.3. Ground wave, which follows the Earth, is a strong function of the quality of the surface along the path. The FCC Web site has some curves for this propagation mode. Beyond about 160 km, HF propagation depends on sky waves reflected from the ionosphere.