Rule sections 73.183 and 73.184 refer to AM groundwave field strength Graphs 1 through 20. As the graphs themselves are not available in the FCC's rules, we make them available here in three versions, PDF, PostScript, and as JPEG images. These graphs are used by broadcast engineers in the preparation of AM broadcast station construction permit and license applications.
Best quality and smallest file sizes will be obtained in most cases using the PDF illustrations of these curves. The PDF graph files can be resized and printed on larger paper (for example, 11" x 17") with excellent quality. The PostScript graphs will reproduce best on PostScript-capable printers.
| Groundwave Curves Sets (zipped file sizes): |
PDF (0.5 MB) | PostScript (0.8 MB) | JPEG (22.8 MB) |
For each graph number (1 through 20), there are two different graph sheets provided. The first graph sheet contains two sets of curves. The bottom set of curves covers distances between 10 to 5000 km from the transmitter site, while the top set of curves covers distances between 0.1 and 50 km on an expanded scale. Electric field strengths in millivolts per meter are listed on the left margin. The numbers on the individual curves represent ground conductivities, with the best conductivity (5000) corresponding to seawater (salt water) and the lowest conductivity (0.1) corresponding to poorly conducting soil. Ground conductivities in the United States typically fall between 0.1 and 30 milliSiemens per meter (mS/m, formerly millimhos per meter, where 1 mho = 1 / ohm) (see the map). The "A" graph sheets (Graph #-A) contain an expansion of the top set of curves, covering distances between 0.1 and 50 km from the transmitter site.
Additional information about AM radio broadcasting is available at the Audio Division on the FCC's website, and at Broadcast Radio Links.
FCC > Media Bureau > Audio Division