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Topic: RF module power limits: Part 15 TL;DR (Read 871 times) previous topic - next topic

Geek Emeritus

May 25, 2019, 08:48 pm Last Edit: May 26, 2019, 04:19 pm by Geek Emeritus
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 certifed. 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 rather more convoluted. PDF page 17:

125/3 X f(mHz)-(21250/3) microvolts per meter measured at 3 meters.
In spreadsheet format:
Code: [Select]

a convenient formula is provided on page 32 of the PDF. They even show the math used to derive it

P (watts) = .3 XE^2, where E is in volts, with an antenna gain of 1 ( The most straightforward antenna design has a gain of 2.14 dbi ). 
In spreadsheet format, corrected for millivolts & milliwatts:
Code: [Select]

Where A2 is the result of the previous spreadsheet formula

Conversion to dbm: 10 X log(mw)
In spreadsheet format:
Code: [Select]

Where B2 is the result of the previous spreadsheet formula

if you toss all that in a spreadsheet, you end up with:

360 mW = 25.5 dbm max legal power at 433 mhz

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 certifed.
  • 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.
All PMs will be deleted unopened due to arrogant argumentative pot stirring Malfoys.
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May 25, 2019, 10:33 pm Last Edit: May 26, 2019, 11:27 am by AWOL
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!

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