Figure R3 of 47 CFR 73.190 of the Commission’s Rules contains a map of the estimated effective ground conductivity in the United States. This data is used to predict the propagation of AM signals across the United States. A higher ground conductivity indicates better AM propagation characteristics. The map shows that the ground conductivity in the U.S. ranges between 0.5 and 30 millimhos (or millisiemens) per meter. The conductivity of seawater is 5,000 millimhos per meter, resulting in the best propagation of AM signals.
Via the zipped file below, you may retrieve scanned map portions of a wall-sized ground conductivity map, which may be printed, trimmed, and assembled into a 43 inch by 69 inch map. There are 48 portions, with map 01w.jpg located in the upper northwest (Washington State). The map portions are scanned for and best printed on 8 1/2 inch by 14 inch paper. (Printing on smaller paper will increase the number of map segments which much be joined.) Together, the 48 map files require 6.8 MB of memory. Separate conductivity maps for Alaska and Hawaii are also included in the zipped file.
The M3 conductivity data is also available as text files. Data for the continental USA is located in the file m3.seq, which also contains updated conductivity data for the Chesapeake Bay and Puget Sound that is not shown on the wall map. Data for Hawaii is contained in the file m3hw.seq. Data for the rest of Region 2 (the Western Hemisphere) is contained in the file r2.seq.
For more information pertaining to AM and FM radio broadcasting and the Audio Division, please call: (202) 418-2700.