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To add to Marshall’s and Rich’s comments I offer a mini study which I posted on another site which is copied in part below in case you didn’t see it. Granted there may have been other factors which attracted the FCC’s attention (such as objectionable content or interefence) but the field strength measurement was the basis for the actions. In several of the NOUOs that I studied the calculated ERP from the antenna was well within the capabilities of commonly used FM transmitters. Based upon my calculations, the citations are not reserved for operations using tens to hundreds of watts.
I do not suggest that you will have trouble with a high field strength, rather that you could have trouble, even with milliwatt levels of ERP. My intent here is to add to the pool of knowledge regarding part 15 FM operation.
Here is what I did:
I was curious about the FM transmitter power levels involved in FCC NOUO (notice of unlicensed operaton) actions so I did a bit of research. The FCC states in the NOUO the measured field strength at a distance. From the FCC site, I selected some NOUO reports for FM and noted the field strength and distance. I then calculated the approximate ERP involved with each of these.
I used the following assumptions to calculate the ERP:
1. The peak free space field strength produced by 11 nanowatts ERP at 3 meters is 250 uV/m.
2. The antenna is a resonant dipole. (Actual antennas may have more or less gain than a dipole so these numbers do not represent the exact transmitter output power.)
3. The field strength increases linearly with distance upon approaching the antenna.
4. The distances and field strengths reported by the FCC were accurate.
I derived the equation
ERP(mw.) = (19.6 x 10^-12) x [ FS(uV/m) x distance(meters) ]^2
to give the approximate ERP. Using this, I calculate the ERP numbers below.
FCC NOUO resulted from these calculated estimates of ERPs expressed in milliwatts:
93., 357411., 6.7, 0.001, 289., 0.703, 577., 120., 0.187