Last Updated on January 13, 2025 by John Berry
Ground wave propagation in the frequency range 10kHz to about 30MHz depends a lot on the characteristics of the ground and sea between the stations. Ground and sea are defined by their magnetic permeability, μ; electrical permittivity, ε, and electrical conductivity, σ. The permeability is normally approximated to that in a vacuum and so needs no manual input. Below 3MHz, the conductivity is the most important.
These characteristics are used to determine which graph to use to determine the value of E, the received field strength, for differing frequencies. These characteristics moderate the relationship between the two variables of frequency and distance and hence determine the ground wave path loss.
Correlation with geography
Ground conductivity and relative permittivity vary as the nature of the soil and water. Both change with geography across a country, region, and continent. The conductivities in mS/m for parts of the United Kingdom (Scotland and England) are shown below. There is rough correlation between conductivity and geography.
Ground around Fort William is upland and mountain with conductivity of 1-3mS/m. Ground around Glasgow and Edinburgh is, by comparison, lowland with a slightly improved conductivity of 4-10mS/m.
Conductivity (and its associated parameter, relative permittivity) is available for most countries in the World atlas of ground conductivities. Some are coarse, others more granular. Methods are available to arrive at a single representative value for a path transiting a mix of ground types and sea.
Values of ground conductivity and relative permittivity
The following summarises the values of ground characteristics worldwide. The characteristics vary according to the nature and moisture content of the soil.
| Relative permittivity, ε | Electrical conductivity, σ | |
| Ground/sea type | ||
| Sea water | 80 | 5 S/m |
| Wet ground | 30 | 10 mS/m |
| Fresh water | 100 | 3 mS/m |
| Medium dry ground | 15 | 1 mS/m |
| Very dry ground | 3 | 0.1 mS/m |
| Pure water | 100 | Very low |
| Fresh water ice | 3 | About 0.015 mS/m |
The above vary little between 10kHz and 30MHz. The exception is freshwater ice where the variation in conductivity is a factor of 10 over the frequency range.
There are two other characteristics: penetration of waves into the soil during propagation, and temperature of the ground and sea. Both are typically accounted for in curves and programs and hence they are not directly input by a user.