5 ms·

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

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.

Aloha 4y ago
It'd have to sit down and do some math, but I'm kinda skeptical that an FM Broadcast band harmonic would land on the air and right next to it.
- Johnythree 4y ago
  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.
  - anthk 4y ago
    UHF and VHF? with an RPI?
    - Johnythree 4y ago
      Any square-wave signal (eg PWM) will have spurs going up to UHF and possibly higher.
stefan_ 4y ago
It's a GPIO with the corresponding mA drive, it can not "seriously jam" anything. The hyperbole is ludicrous.
- Johnythree 4y ago
  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.
  - userbinator 4y ago
    They do that with very carefully optimised equipment.
    - simfree 4y ago
      And when certain atmospheric conditions exist.
      - Johnythree 4y ago
        Except this discussion is about possible interference across town, or even more locally. No need for "certain atmospheric conditions". The point was that very low power signals can go a long, long way.
        simfree 4y ago
        You explicitly brought up long distance communication in this thread: > And even a few micro-watts is sufficient to communicate across town. > Many years ago the record was "1000 miles per milliwatt" using CW.
        Johnythree 4y ago
        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.
mwint 4y ago
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"?
- tb_technical 4y ago
  Basically, the FCC can slap you with an expensive fine when the local HAMs rat you out.
  - connicpu 4y ago
    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
    - Johnythree 4y ago
      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).
      - simfree 4y ago
        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.
kragen 4y ago
i don't think this uses pwm

Aloha 4y ago

It'd have to sit down and do some math, but I'm kinda skeptical that an FM Broadcast band harmonic would land on the air and right next to it.

Johnythree 4y ago
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.
- anthk 4y ago
  UHF and VHF? with an RPI?
  - Johnythree 4y ago
    Any square-wave signal (eg PWM) will have spurs going up to UHF and possibly higher.

stefan_ 4y ago

It's a GPIO with the corresponding mA drive, it can not "seriously jam" anything. The hyperbole is ludicrous.

Johnythree 4y ago
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.
- userbinator 4y ago
  They do that with very carefully optimised equipment.
  - simfree 4y ago
    And when certain atmospheric conditions exist.
    - Johnythree 4y ago
      Except this discussion is about possible interference across town, or even more locally. No need for "certain atmospheric conditions". The point was that very low power signals can go a long, long way.
      - simfree 4y ago
        You explicitly brought up long distance communication in this thread: > And even a few micro-watts is sufficient to communicate across town. > Many years ago the record was "1000 miles per milliwatt" using CW.
        Johnythree 4y ago
        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.

mwint 4y ago

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"?

tb_technical 4y ago
Basically, the FCC can slap you with an expensive fine when the local HAMs rat you out.
- connicpu 4y ago
  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
  - Johnythree 4y ago
    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).
    - simfree 4y ago
      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.

kragen 4y ago

i don't think this uses pwm