RF module power limits: Part 15 TL;DR

I see people posting about RF modules who are not entirely clear on certain concepts. This will be the first of a series of articles about RF modules. In this lesson we clarify the U.S. regulations.


The ARRL page on FCC Part-15 Rules: Unlicensed RF Devices

What we call 433 mhz modules use a frequency band that overlaps a ham radio ( lower case, please ) allocation between 420 & 450 mhz. What we call 915 mhz modules overlap a ham radio allocation between 902 & 928 mhz. The primary user of these bands is the U.S. government, usually the DoD. Amateur radio is a secondary allocation, and hams are licensed users of the band. As such, they are authorized users and they have the right of way over Part 15 devices. If the government or a ham clobbers your Part 15 device; tough luck:

PDF Page 5:

If a Part 15 transmitter does cause interference to authorized radio communications, even if the transmitter complies with all of the technical standards and equipment authorization requirements in the FCC rules, then its operator will be required to cease operation, at least until the interference problem is corrected. Part 15 transmitters receive no regulatory protection from interference
Section 15.5

PDF Pages 5 & 6: Part 15 devices needs to be certified. This process includes verifying that the power output of the combination of the device and the antenna are within limits.

Certified transmitters also are required to have two labels attached: an FCC ID label and a compliance label. The FCC ID label identifies the FCC equipment authorization file that is associated with the transmitter, and serves as an indication to consumers that
4the transmitter has been authorized by the FCC. The compliance label indicates to consumers that the transmitter was authorized under Part 15 of the FCC rules and that it may not cause, nor is it protected from, harmful interference.

Thus, a low power transmitter that complies with the technical standards in Part 15 with a particular antenna attached can exceed the Part 15 standards if a different antenna is attached.

This means that Part 15 transmitters must have permanently attached antennas, or detachable antennas with unique connectors. A "unique connector" is one that is not of a standard type found in electronic supply stores.
Section 15.203

It is recognized that suppliers of Part 15 transmitters often want their customers to be able to replace an antenna if it should break. With this in mind, Part 15 allows transmitters to be designed so that the user can replace a broken antenna. When this is done, the replacement antenna must be electrically identical to the antenna that was used to obtain FCC authorization for the transmitter. The replacement antenna also must include the unique connector described above to ensure it is used with the proper transmitter.

Which describes no module I have seen. You have to be truly stupid to think that a connector will not be "not of a standard type found in electronic supply stores" a week after it is put in use.

Part 15 device RF power out is measured in microvolts per meter at 3 meters. Chances are you will never see the meter that can measure that. Clever manufacturers certify them with inefficient antennas, so they can maximize transmitter power. Any antenna you put on better than the ones that look like a coil spring from a pen puts you over the limit. You may have noticed that the number of people prosecuted for running a module over the power limit is equal to the number of people who died from swimming too soon after eating.

The power limit for spread spectrum 902-928 mHz transmitters is sensibly clear: 1 watt, per PDF page 21.

The power limit for a 433 mHz transmitter is even more clear: there is no 433 mhz ISM band in the USA. T

The bottom line:

  • most RF modules are lacking in the labeling department, but there is nowhere to put all that on a module
  • if it does not come with an antenna, it can not truly be certified.
  • you are probably running a little bit outside the law. even if you have a low gain antenna, if it is not the specific antenna used in the certification process, you are not legal.
  • An RFM22 or 23 is not kosher, if you are not a licensed ham. If you are a licensed ham you need to follow the ID rules when using an RFM22 or 23.

ADDED Feb 11 2021: The Part 15 rules would baffle a Beverly Hills divorce lawyer. I just discovered this:

47 CFR § 15.240 - Operation in the band 433.5-434.5 MHz.

"§ 15.240 Operation in the band 433.5-434.5 MHz.

(a) Operation under the provisions of this section is restricted to devices that use radio frequency energy to identify the contents of commercial shipping containers. Operations must be limited to commercial and industrial areas such as ports, rail terminals and warehouses. Two-way operation is permitted to interrogate and to load data into devices. Devices operated pursuant to the provisions of this section shall not be used for voice communications. "

so, that is why Chinese tracking numbers stop working when your part makes it to the USA. the RFID tag is removed at the entry port.

unless and until a lawyer steps up to clarify things, ( not holding my breath ) or the FCC clarifies things, I will not be using any 433 MHz modules in the US

What we call 433 mhz modules

sp. "433 MHz"

You may have noticed that the number of people prosecuted for running a module over the power limit is equal to the number of people who died from swimming too soon after eating.

No, I have not noticed, but I'll certainly keep my eyes open for either event. Thanks for the tip!