FCC §15.223 – AM Carrier Current Systems (1705 kHz to 10 MHz)
Posted on March 7, 2018
Below is a post I made in another thread here on Pt15us. However the subject of that thread would make it hard to locate for anyone searching for information on this topic. This re-post to my blog should be easier to find.
From §15.223: The field strength of any emission within the band 1.705-10.0 MHz shall not exceed 100 microvolts/meter at a distance of 30 meters. However, if the bandwidth of the emission is less than 10% of the center frequency, the field strength shall not exceed 15 microvolts/meter or (the bandwidth of the device in kHz) divided by (the center frequency of the device in MHz) microvolts/meter at a distance of 30 meters, whichever is the higher level. For the purposes of this section, bandwidth is determined at the points 6 dB down from the modulated carrier. The emission limits in this paragraph are based on measurement instrumentation employing an average detector. …
The text shown in bolded characters above permits the greatest field strength for a typical, legal AM transmit system permitted by this paragraph — which at a distance of 30 meters is 15 µV/m, not 100 µV/m.
Unfortunately, there are few, if any, consumer-level AM broadcast receivers (or receive locations, for that matter) where a signal strength of 15 µV/m would provide an acceptable/useful signal-to-noise ratio in the audio output of that receiver. Even if receivers could do that, a legal system under §15.223 could not provide a useful coverage area having a radius of a mile or more, unless all or most receivers in that area were located within a 30-meter distance from a radiating conductor of the a-c power lines.
AM broadcast receivers that are powered directly from the a-c line are close enough to it to receive a much higher, legal field strength than 15 µV/m, because they are in the extreme near field of the radiating conductor(s) of the a-c power line they are plugged in to. But that higher field drops very rapidly to below the noise floor when a battery-operated receiver such as a car radio or “Walkman” is located more than several meters away from the nearest radiating conductor of the a-c power lines.