41–50 of 89 posts
this is completely illegal.
So? It's a proof of concept. Tinker away, laws be damned.
Mentioning that even this proof of concept is illegal is useful, as it isn't mentioned in the article. This is so easy to make that practically anyone could do it, which makes this very relevant.
Earlier quoted context omitted.
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).
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.
Earlier quoted context omitted.
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 th…
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.
Earlier quoted context omitted.
So? It's a proof of concept. Tinker away, laws be damned.
Until an angry Radio Inspector arrives on your doorstep having spent all day driving around your town with a spectrum analyzer to track you down. When you get to attend court, the judge will award the standard fines (and possible jail time) but also the cost of the time spent by the Radio Inspector in traveling to your town.
Earlier quoted context omitted.
It's a GPIO with the corresponding mA drive, it can not "seriously jam" anything. The hyperbole is ludicrous.
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.
Author here. This code no longer works on new Pi's (it does a lot of low level very hardware stuff from userspace without a proper kernel driver). The code was written in a 24 hour Hackathon, so is very far from maintainable. However there are other projects to do the same which do work on new Pi's. There are ways to do a full software defined radio from a pi by using the I2S hardware, and that can transmit GPS, 433…
> "using some filters on the output" While it's relatively easy to remove harmonics, it is pretty much impossible to remove the broad comb of spurious signals either side of the carrier. Which why transmitters are not designed this way in the real world.
If we were talking about AM I would assume you mean the audio signal isn't bandlimited and so high-frequency audio components get upconverted by the carrier frequency, but I don't know what the spectrum of an FM signal looks like. Are we talking about a broad comb that results in an analogous way from sudden changes in the frequency? Do they go away if you vary the frequency of a 50%-duty-cycle square wave smoothly, like maybe with second- or third-order continuity, instead of suddenly?
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 t…
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 t…
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"?
[1] https://www.digikey.com/en/products/detail/seeed-technology-...
[2] https://www.viavisolutions.com/en-us/products/interferencead...
If you're generating RF with homebrew stuff, get a spectrum analyzer.[1] They're much cheaper than they used to be. Then you can find out if you're blithering all over the RF spectrum, and how badly. It's better to find it out yourself before someone from a cell phone company shows up, spectrum analyzer in hand. Cellular providers call this "spectrum clearing". There are now automated systems to help with this.[2] [1…