One of the basic inputs to antenna modeling programs are the ground conductivity and relative dielectric constant. Frequently however, modelers have no real idea what their particular ground characteristics are. It is possible to make useful soil parameter measurements at HF which are sufficiently accurate for modeling purposes. The nice thing is that these measurements are not very difficult requiring only a very simple mechanical assembly, say a rod and a piece of screen, and an impedance bridge like an MFJ259 or similar units. The following file has a detailed discussion of this kind of measurement: ground parameter measurements, 1 MB . I recently gave a talk at the Dayton Hamvention. Here are the slides: Dayton 2005 slides, 220 kB.
Sometimes it is handy to know the skin depth and wavelength in a particular soil. Skin depth and wavelength in soil, 70 kB.