FCC Regulations for Unlicensed FM Microtransmitters...Not Very Specific...

[Tom_]

The FCC rules for Unlicensed Operations of Microtransmitters contain inadequate information resulting in possible inadvertent and unintentional interference:

For example, from FCC Public Notice 14089 (released 7/24/1991):

Maximum Effective Radiated Power is
“0.01 microwatts…Rule Actually Specifies a limit of 250uV/meter measured at 3meters.”

Weakness: What if you’re on a hill? You might get 250uV/m at 6-9 meters.

Approximate Maximum Coverage Radius is “200 feet”

Weakness #1: What does “Approximate” mean?

Weakness #2: What in uV/m at 200 feet? Needs to be more specific. Or, how about giving the maximum distance for the 60dB contour?

Bandwidth:

20kHz, from “Understanding the FCC Regulations for Low-Power, NON-Licensed Transmitters” OEC Bulletin No. 63 at the FCC web site

Missing:

1 - No height limit, e.g. 2nd or 3rd floor.
2 – Nothing about a building at the top of a hill.
3 – Nothing about whether it is legal to broadcast the signal directionally in order to prevent frequency from staying within 200' downhill.

With the proliferation of the sale of portable listening devices and microtransmitters, I would not be surprised if OET Bulletin No. 63 is revised to take these and other deficiencies in the rules into effect.

However, are these concerns legitimate or not?
Comments? Suggestions?

Why not sell directional unlicensed low power FM transmitters for residences on hills (that is, to push the signal uphill and less downhill)? That's what I need. Otherwise people on hills will be in violation even w/ licensed devices.

 

[ironbear]

> However, are these concerns legitimate or not?
> Comments? Suggestions?

I wouldn't worry much about exact measurements while operating any certified, very low powered transmitter in the real world. A hill or 2nd story mounting wouldn't effect field strength over the required measurement distance of 3 meters, and the FCC knows that these signals reduce in strength considerably as you get even a short distance from the transmitter.

Bottom line...250uV/meter measured at 3 meters specifies lab test conditions you'd expect to meet when getting a device certified under Part 15. Unless you happen to have a calibrated FIM (field intensity meter) and intend to get your own special design certifed, this stuff isn't of much actual use. “0.01 microwatts" is very low indeed, so low it's not measurable with standard Wattmeters, and that's why the FCC specifies the limit as a field strength measurement, BTW.

 

[pbolyn]

I have observed that some of the wording in these notices and bulletins seems to eventually get interpreted as being part of the rules. For example, the rule is 250 uV/m at 3 meters. "Approximate Maximum Coverage Radius is “200 feet” is NOT a rule. It is just some technical guy's contribution trying to clarify the actual rule for people who are maybe less technically inclined. This stuff tends to muddy the waters. When in doubt, go by the rules, not the "clarification". If you have a dandy antenna and location and somehow get 10 miles range (exagerated for effect) and the field intensity of your signal is 250 uV/m at 3 meters, then you are legal. That is simply what the rule says. The rule specifies signal strength at 3 meters only, not range.

Phil B


> The FCC rules for Unlicensed Operations of Microtransmitters
> contain inadequate information resulting in possible
> inadvertent and unintentional interference:
>
> For example, from FCC Public Notice 14089 (released
> 7/24/1991):
>
> Maximum Effective Radiated Power is
> “0.01 microwatts…Rule Actually Specifies a limit of
> 250uV/meter measured at 3meters.”
>
> Weakness: What if you’re on a hill? You might get 250uV/m
> at 6-9 meters.
>
> Approximate Maximum Coverage Radius is “200 feet”
>
> Weakness #1: What does “Approximate” mean?
>
> Weakness #2: What in uV/m at 200 feet? Needs to be more
> specific. Or, how about giving the maximum distance for the
> 60dB contour?
>
> Bandwidth:
>
> 20kHz, from “Understanding the FCC Regulations for
> Low-Power, NON-Licensed Transmitters” OEC Bulletin No. 63 at
> the FCC web site
>
> Missing:
>
> 1 - No height limit, e.g. 2nd or 3rd floor.
> 2 – Nothing about a building at the top of a hill.
> 3 – Nothing about whether it is legal to broadcast the
> signal directionally in order to prevent frequency from
> staying within 200' downhill.
>
> With the proliferation of the sale of portable listening
> devices and microtransmitters, I would not be surprised if
> OET Bulletin No. 63 is revised to take these and other
> deficiencies in the rules into effect.
>
> However, are these concerns legitimate or not?
> Comments? Suggestions?
>
> Why not sell directional unlicensed low power FM
> transmitters for residences on hills (that is, to push the
> signal uphill and less downhill)? That's what I need.
> Otherwise people on hills will be in violation even w/
> licensed devices.
>
<P ID="signature">______________
Phil B
</P>