This program performs calculations of the theoretical inverse distance field strengths in the vicinity of AM broadcast radio stations, using the graphs in Figure 8, Section 73.190 of the FCC's rules. This program will calculate the theoretical field strength generated by the AM station at a distance of 1 kilometer (or one mile if units of feet and miles are selected) at the specified effective radiated power (ERP). It will also find the theoretical field strength using a reference ERP of 1 kW. Calculations may be made in metric or English units, or in electrical degrees (360 electrical degrees = 1 wavelength).

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.

The inverse distance field is the electric field strength generated by the AM station at the assigned station frequency. Units of measurement are in volts per meter (V/m) or, as in this program, 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 may be used by broadcast engineers to predict whether a proposed operation will meet the Commission's minimum efficiency requirements for AM broadcast stations (see 47 CFR 73.189 and Figure 7, 47 CFR 73.190), prior to conducting actual measurements of field strength. It can also serve 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 (see Section 73.45 and Section 73.189(b)). Applications for AM broadcast stations which 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 being generated by the two or more towers comprising the 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) may be obtained by retrieving the Adobe PDF version of Section 73.190 (may take some minutes to download -- contains several graphs).

Alternatively, enlarged versions of Figures 7 and 8 may be retrieved 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 3 sections)

• 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)