## Monday, June 20, 2011

### Field Strength Calculations: Ground Conductivity

Ground (also called soil) conductivity plays a huge role in how far the mediumwave signal travels during the daytime. Lesser known to many, station frequency is also a factor, and maybe more of one than you would think. Though one can argue successfully that frequency is not a factor in the formula for calculating received signal strength, it indeed becomes relevant as you will see. In this series, let's explore ground conductivity and station frequency and see how they relate to "how far you can hear" on the mediumwave band during the daytime. We will end with a method to calculate approximate field strength for stations of interest.

Many years ago, ground conductivity measurements were compiled into a map titled "Estimated Effective Ground Conductivity in the United States" (Figure M3) by the FCC. This map is used for the allocations planning for placement of MW stations in the United States. The map presents optimistic ground conductivities and is used when measured conductivity is not available. This information has been used and accepted since it was compiled in 1954.

Soil conductance is measured in siemens per meter but most generally shown in millisiemens per meter. This is the mS/m designation you see in the accompanying chart. The siemens (symbolized S) is the Standard International (SI) unit of electrical conductance. The old term for this unit is the mho (ohm spelled backwards). Conductance (mho) is of course the opposite of resistance (ohm). As you can see, salt sea water provides the best conductance by far (5000 mS/m). The higher the value the better. Average soil runs 6-8 mS/m. Find your location on the map and see what your local conductance value is. Notice also that a distant station's receive path may transition across more than one zone.

Equally important, the FCC also produces a series of charts known as the "Ground Wave Field Strength Versus Distance" graphs. These 20 graphs in .PDF form are grouped by mediumwave channels in the 540-1700 KHz range, and allow prediction of received signal strength by cross-referencing the distance to the receiving location with the ground conductivity factor between you and the station. These charts cover soil conductivity ranges of 0.1 mS/m to 5000 mS/m. They are still in use today. The only working link I have for them is:

FCC Ground Wave Field Strength Versus Distance Graphs