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Comments have been made on various Part 15 boards (even recently) to the effect that this particular setup as described for Mr. Gaule using an FCC-certified Part 15 AM transmitter could not have produced the 1,800 µV/m field that the FCC measured at 175 meters.
The graphic below shows a NEC4.2 analysis of a system possibly similar to the one used by Mr. Gaule. The top of the antenna system is elevated 7 meters above the surface of the earth. A wire path representing the a-c wiring in the structure connects the transmitter chassis to a 3 meter buried conductor, which represents the utility ground rod at the service entrance.
The analysis shows that an r-f power of 60 mW applied to the input of the loading coil produces a field intensity of 1.8 mV/m (1,800 µV/m) at a horizontal distance of 0.175 km (175 meters). That field intensity was shown for that distance in the FCC NOUO issued to Mr. Gaule, for this 1700 kHz frequency.
The two outlined areas shown in the left pane of the graphic give some of the relevant parameters of this study.
So it does not appear to be unpredictable or mysterious that a system such as this could produce the field measured by the FCC at that distance — whether or not its transmitter was FCC-certified for Part 15 AM.
Right clicking on the image and choosing “View Image” will bring up a larger-sized image (in Firefox, anyway).