This is so bad, and dangerous. It outputs pwm from an unfiltered digital output, therefore generating a hell of harmonics on a wide set of frequencies, not just the one it's set to operate on. It can seriously jam not only broadcast FM transmission but also other services. Beware that airband is very close; any jamming on those frequencies can turn in huge fines if not landing the perpetrator straight to jail.
Want to test it? Fine, but never ever connect an antenna to it.
In real life, what would happen to a legitimate clueless hobbyist who tries this (with no ill intent or knowledge of what's happening) and gets caught? I'm assuming the real response is closer to "hey, don't do that, read this PDF" than it is to "straight to jail"?
A 20cm antenna driven by a raspberry pi's current limited GPIOs won't be powerful enough to get picked up by anything more than a few dozen meters away, especially not powerful enough to get in the way of actual signals being transmitted by a more powerful transmitter
A 20cm antenna will be a 1/4 wave at 375 MHZ (and a half wave at 188Mhz, etc), and thus will be a very efficient radiator at many different frequencies.
Even a tiny signal can be enough to block the operation of a nearby police repeater (which is also trying to hear weak signals).
Perhaps public safety shouldn't be entirely beholden to Motorola rebranding old garbage (ancient TDMA networks being sold as new P25 compliant hardware for tens of millions of dollars) that is then sold as new.
Modern protocols like 5G New Radio that have better propogation at the fringes of connectivity would serve life critical applications with improved coverage and significantly more intelligible voice quality than what the current public safety networks are capable of doing.
If you look at the telephone/power poles in your neighborhood you will see a device with two antennas hooked into power every few blocks. This is a recieve only amplifier that has to be used to amplify these weak signals from handheld public safety radios that use protocols like TDMA with poor interference resistance and no error correction.
If the wanted signal is very weak (eg from a distant aircraft using VHF AM), and there is an interfering signal on the same channel, the emergency communications will be distrupted.
Aircraft using the VHF band with amplitude modulation is going to perform much differently than a police repeater and it's local upstream only amplifiers interacting with low wattage fixed handhelds and medium wattage car mounted mobile radios.
Pivoting topics is a bad faith conversational tactic FYI, and doesn't follow HN's guidelines. I encourage you to fully read the guidelines linked at the bottom of the page.
Nevertheless, the threat is to public safety communications. And not all Public safety use TDMA (or whatever). Aircraft communications is probably the most vulnerable, but so are HF/VHF marine operations, military and a host of others.
TDMA amplifiers could well be a problem in your country, but the whole point of the discussion is that transmitting a wide band of RF hash will certainly be detrimental to thousands of other radio users, whether or not they are using a TDMA public network.
Amplifiers are often protocol agnostic. Whether your amplifying TDMA, Trunked radio, CDMA etc.
Aviation needs to modernize, I have zero sympathy for the aviation community getting constant hallpasses on ancient altimeters that lack filters to reject RF from nearby frequencies, leaded avgas, and abysmal working conditions for those stuck in the industry.
The FAA dragging their feet in the USA trying to act like they should never have to test how interference affects hardware on planes is insane, they waited till well past when 3.5Ghz went into cellular use to make any effort to performance test altimeters for band conformance. The FCC's timelines are long, the FAA had at least half a decade heads up of when the 3.5Ghz auction would occur, and additional time after the auction before Verizon and AT&T put the spectrum into service.
We have waited 25 years past when everyone else stopped using leaded gas to get rid of leaded Avgas[1].
As a Seattleite who grew up with families made up of Boeing Engineers, the problem of vulnerable radio transmissions from aircraft could be resolved with a network of reception sites just like Gogo uses at a much higher frequency allocation. This problem is just not a priority for the Aviation community, they would rather not invest in better equipment and infrastructure.
Aircraft use AM on VHF because it is still the most efficient mode.
Unlike FM it doesn't suffer from the "Capture Effect" so you can tell if a weaker signal is trying to get through on a busy channel. And unlike FM it doesn't suffer from the Threshold Effect where weak signals suddenly disappear into the noise.
And of course Digital modulation is ruled out by both of the above considerations.
The only mode which is more efficient (for weak signal communications) is Single Sideband, and that can't be used in aircraft due to Doppler shift.
It is true that local reception sites can help, but they are not possible over water. Perhaps one day satellite systems might finally replace aircraft AM. The problem at present is the cost for fitting to light aircraft.
The problem with QPSK in particular is a loss of phase lock in marginal conditions, or with interference.
It would require higher complexity, with no improvement for marginal signals.
One possible improvement would be "Synchronous AM" (eg 1/2 of a QAM signal) but that will also lose phase lock in weak signal conditions, eg when it is most needed.
I know nothing about how this all works, how do people know who did it / where it came from? I presume its possible to get a rough location but in a built up area I'd imagine its pretty much impossible to find the person that did it unless it's left running right?
If you're interested, Hams do an activity called a "Foxhunt" where they're trying to triangulate the position of a broadcasted signal. It's sort of like wargaming, but for Hams.
This, in conjunction with whoever owns the plot of land the offending device happens to live on, determines who is to blame when someone is broadcasting illegally.
If you are blocking an essential services channel (eg Aircraft, Police, Ambulance, military), the authorities can get a triangulation (within meters) on you in just a few minutes.
Radio Amateurs routinely communicate around the world with just a few milli-watts. And even a few micro-watts is sufficient to communicate across town.
Many years ago the record was "1000 miles per milliwatt" using CW.
Except that both can be true. A few millwats can reach across town, or across the world. The original claim was that "weak signals don't go very far", or some such.
It's not so much "harmonics of the FM band", but the wide band of spurs either side of the transmission.
The problem is that the unfiltered output of this device contains a wide range of spurs and harmonics which will radiate right across the VHF and UHF bands.
Plus the antenna (a few feet of wire) will have multiple resonances across the same range.
It will radiate hundreds of signals, and the odds of them interfering with a nearby essential service will be very high. And note that the essential service receiver is capable of hearing signals in the micro-watt range and is routinely trying to receive them.
