FCC OET Bulletin 63 (link below) is titled UNDERSTANDING THE FCC REGULATIONS FOR LOW-POWER, NON-LICENSED TRANSMITTERS.
Page 29 of this document states:
What is the relationship between "microvolts per meter" and watts?
Watts are the units used to describe the amount of power generated by a transmitter. Microvolts per meter (µV/m) are the units used to describe the strength of an electric field created by the operation of a transmitter.
A particular transmitter that generates a constant level of power (watts) can produce electric fields of different strengths (µV/m) depending on, among other things, the type of transmission line and antenna connected to it. Because it is the electric field that causes interference to authorized radio communications, and since a particular electric field strength does not directly correspond to a particular level of transmitter power, most of the Part 15 emission limits are specified in field strength.
Page 29 goes on to develop an equation to calculate the power needed to produce a specific field intensity at a distance of three meters, assuming a transmit antenna system with a gain of zero dBi (an isotropic radiator):
P = 0.3E^2 where P= power in watts and E = field intensity in V/m
Inserting the field limit of FCC §15.239 on the right side of the equation:
P = 0.3(0.00025)^2 = 0.000 000 018 750 watts, or 18.75 nanowatts
Discussion:
There are many transmitters sold for Part 15 FM use that are rated for hundreds and even thousands of times more output power than legally can be radiated under FCC §15.239.
The FCC has issued citations to some unlicensed operators where it can be calculated that the radiated powers used fall into the range of those produced by transmittters commonly sold and used for Part 15 FM. Here is summary of some of those:
This information is not provided or intended to deter anyone from using/enjoying Part 15 FM, but to give some perspective on the operational realities permitted by FCC §15.239.
Here is a link to FCC OET Bulletin 63:
https://www.fcc.gov/Bureaus/Engineering_Technology/Documents/bulletins/o...