Are there some technical errors in this FCC document? - Page 1
Forum section: electronics / rf-microwave

author: Ben321
author_rank: Frequent Contributor
timestamp: March 11, 2022, 07:23:58 am
content: I found this official FCC document while looking for the the legal power limits for part 15 unlicensed transmitters.https://docs.fcc.gov/public/attachments/DOC-297510A1.pdfBut it appears to have at least one error. For FM it says:QuoteI'm guessing that power specification of 0.01 uW is an error by whatever FCC employee typed this documents, because radio transmissions, in order to even be slightly useful, need to be broadcast at a power level of at least 1 uW, while 0.01 uW is literally 10 NANOwatts. This is such a low power level, that a typical FM radio receiver wouldn't be able to get a usable signal beyond an inch at most. If it instead said 0.01 mW, that would make a lot more sense, as that would be 10uW, which is a reasonable power level for low power transmission that would get you maybe a few feet radius from the transmitter antenna where there would be a usable signal level.Also it seems there error there specifying signal power at the antenna versus signal intensity at a distance. Signal power is stated before the parentheses, and is set only by the electrical output of the device to the antenna (and is measured in watts), while signal intensity (stated in the parentheses) is a property of the signal at a certain distance from the antenna (and is measured in volts/meter) and it is set not only by the power delivered to the antenna but also by the length of the antenna relative to a quarterwavelength of the signal frequency. It seems an error that they give 2 separate specifications for legal output from the transmitter. Either it's legal based on power, or intensity. Listing both doesn't make sense.And for AM, it says:QuoteSo in one case it says 50mW output, but then also goes on to say 100mW sent TO the final stage (not output). This seems strange. Does this mean that the final stage is expected to cut the output power in half, to reduce the 100mW sent to the final stage down to 50mW to be output from the final stage? Also why would it matter what power was sent to the final stage at all? FCC rules are designed to prevent radio transmitters from talking over each other so only the actual output would seem to matter, not the RF power levels present inside the device itself (which if designed properly will be shielded with a metal chassis to prevent leaking of the RF signal from any of the pre-output stages). So again this document doesn't make sense here in what it's saying.Or maybe I'm just misunderstanding it. Could someone here explain what these statements in the document mean?

author: radiogeek381
author_rank: Regular Contributor
timestamp: March 12, 2022, 06:20:37 pm
content: Hmmmm... there will be lots of answers here.But one thing you might consider is that July 24, 1991 is pretty far back. The governing rules are in CFR 47 part 15 (take a look at section 15.239.)That said, the limit in that section (as of March of 2022) is 250 uV/m at 3 meters -- just what it was in the note. This section does not frame things in terms of power or antenna gain.Finally, I think the suggestion that 10 nW is too small to be practical is not quite right. If we assume 50 ohm input, 10 nW *at the antenna* works out to 700 uV which is pretty generous. 10 nW is, after all, -50 dBm. Sure we don't see that power 100 meters away, but we've still got 40 dB or more that we could give up before the signal disappears.Converting 250uV/m at 3 meters to uV at the input to the receiver is not simple, as it requires some knowledge of the antenna gain. But, at least by this back-of-the-envelope calculation, the numbers in the regulations don't preclude reception at a modest distance.So no, I don't think it likely that the document was in error as far as the FM requirements go.https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15/subpart-C

author: ejeffrey
author_rank: Super Contributor
timestamp: March 15, 2022, 03:43:21 am
content: 250 microvolt/meter at a distance of 3 meter is about 10 nanowatts. The formula to convert field strength to power is just A * (V/m)^2/Z0. For A = 4 * pi * (3 m)^2 and Z0=377 ohm, that gives 16 nanowatt. That would be for an (unphysical) isotropic radiator, for a typical short dipole as you would expect in a portable FM transmitter a power of about 10 nanowatt would give a maximum field strength of about 250 uV/m at 3 m range.Personal FM transmitters are not intended to go more than a couple of meters and they are not meant to operate in conflict with a licensed station, they only have to overcome the background noise and they are supposed to fade into the background for anyone except the person using it. 10 microwatts would be far too much power and could be received by an ordinary receiver at a distance of 10s of meters.