The inverse distance field is the electric field strength generated by the AM station at the assigned station frequency. The units of electric field strength used here are millivolts per meter (mV/m). Field strength diminishes with increasing distance from the AM station (hence the "inverse" relationship of field strength to distance).
Figure 8 of Section 73.190 is used by broadcast engineers to determine whether a proposed or modified facility will meet the Commission's AM station minimum efficiency requirements (see 47 CFR Section 73.189 and Figure 7, 47 CFR Section 73.190), prior to conducting actual measurements of field strength. It also serves as a basis against which measurement data can be compared. Users are cautioned that this program does NOT determine whether a particular result meets the Commission's minimum efficiency requirements (refer to Section 73.45 and Section 73.189(b)). Applications for AM broadcast stations that do not meet the minimum efficiency requirements generally will not be accepted.
Measurements of the inverse distance field are used to determine the circularity of nondirectional AM stations as well as directional AM stations operating in nondirectional mode (only one tower operating). If the measurements show a consistent field strength at a given distance at all angles from the tower, then the AM pattern is circular. For directional stations, comparison of measurements made in directional mode can be compared to measurements taken during nondirectional operation. Because the "close in" measurements (which are taken within 3 km of the transmitter site) are minimally affected by changes in ground conductivity or seasonal variations, this comparison will show the actual directional pattern generated from the two or more towers comprising the AM directional antenna array.
The graphs contained in 47 CFR 73.190, including Figure 7 (minimum vertical height of AM broadcast station antennas) and Figure 8 (effective field at 1 km for 1 kW) can be obtained by retrieving the PDF version of Section 73.190 (includes several graphs). Alternatively, you can retrieve enlarged versions of Figures 7 and 8 as GIF images to your browser:
- 8 1/2 by 11 inch Figure 7
- 13 by 18 inches Figure 7 (print
using 8 1/2 by 14 inch paper in landscape mode, then join
- 8 1/2 by 11 inch Figure 8
- 13 by 18 inches Figure 8 (print using 8 1/2 by 14 inch paper in landscape mode, then join 3 sections)
Note on Figure 8 results for nondirectional AM stations: Where the average length of the ground system radials is less than 1/4 wavelength, or if the number of ground radials is more than 90 but less than 120, this program will apply the appropriate correction factors and calculate the corrected theoretical field strengths.
Additional information about AM broadcast radio stations is available at the Audio Division on the FCC's website.
Updated: February 5, 2014