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
Be sure to get the .PDF versions.
How well a mediumwave transmitter "gets out" is not only dependent on its power, frequency, and the ground conductivity between it and you, but also on the ground condition at its location. The following is a quote from the Standards of Good Engineering Practice Concerning Standard Broadcast Stations (550-1600 kc.), 1939, and is still relevant today:
"The ideal location of a broadcast transmitter is in a low area of marshy or 'crawfishy' soil or area, which is damp the maximum percentage of time and from which a clear view over the entire center of population may be had... The type and condition of the soil or earth immediately around a site is very important. Important, to an equal extent, is the soil or earth between the site and the principal area to be served. Sandy soil is considered the worst type, with glacial deposits and mineral-ore areas next. Alluvial, marshy areas and salt-water bogs have been found to have the least absorption of the signal."
All well and good. Our transmitter is well-located, emitting a good signal traveling over perhaps many kilometers or miles to our receiving location. We either hear it or we don't depending on the natural attenuation decay between us and the transmitter. But just how do we predict the outcome? How do we (abstractly) measure a mediumwave signal's strength at the receiving end? We will use these tools and others to find out.
Next up: Measurements