The FCC and unlicensed FM transmissions

[radio8z]

Hello all,

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 reports 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 (effective radiated power from the antenna) for 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.

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 calculated the ERP numbers below.

FCC NOUO resulted from these calculated ERPs expressed in milliwatts:

93.
357411.
6.7
0.001
289.
.703
577.
120.
0.187

Note that four of these are well within the capabilities of commonly used unmodified FM transmitters.

Just thought some of you might be interested.

Neil


PS I edited this. I slipped a decimal typing the data. The 703 mW entry is really .703 mW. Neil
<P ID="edit"><FONT class="small">Edited by radio8z on 03/04/06 03:11 PM.</FONT></P>

 

[dickfry]

> Hello all,
>
> 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 reports 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 (effective
> radiated power from the antenna) for 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.
>
> 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 calculated the
> ERP numbers below.
>
> FCC NOUO resulted from these calculated ERPs expressed in
> milliwatts:
>
> 93.
> 357411.
> 6.7
> 0.001
> 289.
> .703
> 577.
> 120.
> 0.187
>
> Note that four of these are well within the capabilities of
> commonly used unmodified FM transmitters.
>
> Just thought some of you might be interested.
>
> Neil
>
>
> PS I edited this. I slipped a decimal typing the data.
> The 703 mW entry is really .703 mW. Neil
>

Without knowing how far the meter was from the antenna when the reading was made by the FCC agent, there is no way to know anything about the transmitter output power, other than it was greater than the numbers would indicate.

The FCC doesn't grab a ladder and climb up on the roof. They measure 3 feet from the nearest accessible property line, at ground level. That's a safe way if they have to go to court, because it will always be less than the actual reading that might be obtained 3 feet from the antenna.


-Jailhouse Dick

 

[radio8z]

Hello Dick,

> Without knowing how far the meter was from the antenna when
> the reading was made by the FCC agent, there is no way to
> know anything about the transmitter output power, other than
> it was greater than the numbers would indicate.

Correct, except the power could also have been less not knowing the distance. The NOUOs report both the field strength and the distance of measurement in most cases. I don't know their distance measurement methods (laser ranger, GPS, calibrated eyeball, etc.) and all I had to use was their data. My calculations are only as good as these data. Good engineering practice is to report data to the accuracy of the instruments and not beyond. They report the FS to 1 uV and the distance to .1 meter, but I don't know that the data are that accurate. If their data were off by a factor of 2 in the same directon (worst case) on both the measurements, it would affect my calculation by a factor of 16. If this were the true range of error, then the range of error on the .703 mW. data point would be .044 mW < ERP < 11.2 mW. Granted this is hypothetical but it does give an indication of the equation's sensitivity to measurement error.

> The FCC doesn't grab a ladder and climb up on the roof. They
> measure 3 feet from the nearest accessible property line, at
> ground level. That's a safe way if they have to go to court,
> because it will always be less than the actual reading that
> might be obtained 3 feet from the antenna.
>
>
> -Jailhouse Dick

Also correct. If the field is greater than 250uV/m 30 meters away, it is greater than this at 3 meters as you state and the exact number is not needed at 3 m. In my data set, all FS were greater than 250 uV/m and all distances were greater than 3 meters so the citatons appear defensible. The NOUOs are based on this and not the ERP.

Another way to look at this is to reverse the calculaton and start with .187 mW. ERP as the given. This will theoretically produce a FS = 916 uV/m at 106.6 meters (one of the data points) and this exceeds 250 uV/m at 3 meters. (The FS will exceed 250 uV/m out to about 390 meters.) In this case I conclude a FCC citation is possible with sub milliwatt ERP.

I did this paper experiment to determine if the FCC actions resulted from ERPs of kW, Watts, mW, etc. Subject to the conditions discussed, it appears that mW. ERP levels were involved.

Neil