There is a lot of discussion online about whether using Pifm is okay because it is low powered, or whether it is very bad because the square waves cause a lot of interference outside the FM band.

With something along the lines of a 30cm antenna, is that harmonic interference really noticeable? Similarly, regardless of how bad the interference is, have there been any cases of someone being contacted by the FCC or similar organization in response to complaints of interference?

3 Answers 3


Flying "under the radar" is nebulous at best. You are correct in that FCC is called in on interference. It normally does require a complaint (which I assure you that you don't want to deal with). However, with low power such as you intend, the chances of you interfering with an authorized broadcast are low. I was involved in a situation (years ago where a 100KW induction heater (10 kHz) had been reported as interfering in the AM broadcast band. The reduction in interference was approved by the FCC, problem solved. No penalties to the offending party. Perhaps go about your property, and those of the neighborhood and see if you can detect FM broadcast of your device. Being responsible party you have already indicated your interest. Make sure that no local emergency broadcasts by your neighbors (Home Alert necklace, etc are endangered). Go forth and sin no more

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    My recollection is that the first harmonic of a square wave is about a third as powerful as the primary at three times the frequency -- that's a heck of a lot of power being transmitted being transmitted on a frequency that your neighbors aren't using. If you're transmitting towards the upper end of the FM spectrum, that harmonic lands on a band marked "aeronautical radionavigation".
    – Mark
    Jul 3, 2014 at 7:26
  • Is "a third as powerful" really "a heck lot of power" in this scenario?
    – andrewmh20
    Jul 3, 2014 at 15:58

There is no doubt that operation is illegal. Regulations vary around the world, but in most countries operating unlicensed radio transmitters is illegal. In many places it is OK to operate low power equipment which is approved under a class licence. WiFi and radio microphones fall into these categories. CB radio and model control are also legal, even though using higher power, but only in approved bands.

There is no way Pifm would qualify, because of the out of band emissions.

Having said this, the chances of being caught and prosecuted is low.

  • do you have any information about "out of band emissions" relevant to the question? Harmonics? This would be good information
    – Marla
    Jul 3, 2014 at 1:13
  • @Marla Obviously I have not tried Pifm, but have looked at the code. There would, at the least be significant harmonic content. Without any filtering there would also be a quite wide sideband components which would cause adjacent channel interference. Just attaching an antenna to the Pi (even without Pifm) would cause a lot of noise to be broadcast.
    – Milliways
    Jul 3, 2014 at 3:24
  • "in most countries operating unlicensed radio transmitters is illegal" -> This is false. For example: "legal in most countries in the European Union" "the UK...permits unlicenced use of devices" "Canada permits transmitters" "the United States...specifies that no license is needed" "In Japan, no license is needed" -- Etc. However, all these places limit the broadcast power, and although how that is determined varies from place to place and the pi likely can exceed it in many of them. Slightly.
    – goldilocks
    Feb 24, 2017 at 7:36

Its 100% illegal, that's kind of like asking if its illegal to drive down the street without a seatbelt if you're only going 5 miles per hour. The answer is, if you get caught its illegal, if you don't get caught you won't get punished. I've ran PiFM for a month straight without getting caught, but run at your own risk. If you're getting caught chances are you were interfering with some pretty important government stuff, as PiFM is a really messy broadcaster that spills over onto lots of frequencys. So use good judgement.

  • "if you get caught its [sic] illegal" is nonsense. You are either complying with the law or you are not. This sounds like someone trying to rationalize behavior that they knew full well was illegal. Feb 20, 2022 at 14:45
  • @ElliotAlderson The point I'm making is its illegal, but if a tree falls in a forest and nobody's around does it make any noise?
    – Kovy Jacob
    Feb 20, 2022 at 14:46
  • Yes, if a tree falls in a forest it makes a sound. What you seem to be saying is that "it is OK to interfere with my neighbor's radio communications as long as I don't get caught". These regulations exist for a reason. Feb 20, 2022 at 15:08
  • @ElliotAlderson Personally I don't think it makes a sound, but notwithstanding, as long as you make sure you aren't operating around any operated frequencies, it isn't legal, but you aren't bothering anyone.
    – Kovy Jacob
    Feb 20, 2022 at 15:23
  • And you can be certain you aren't bothering anyone...how, exactly? These regulations exist for a reason. Feb 20, 2022 at 16:48

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