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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?

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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 '14 at 7:26
Is "a third as powerful" really "a heck lot of power" in this scenario? – andrewmh20 Jul 3 '14 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.

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do you have any information about "out of band emissions" relevant to the question? Harmonics? This would be good information – Marla Jul 3 '14 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 '14 at 3:24

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