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Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

icrobotics.co.uk

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Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#31

Post by Johnythree »

esskay wrote: post #26
tb_technical wrote: post #25

Earlier quoted context omitted.

Basically, the FCC can slap you with an expensive fine when the local HAMs rat you out.

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 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.

Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#32

Post by wrycoder »

BenjiWiebe wrote: post #23
msla wrote: post #8

Earlier quoted context omitted.

> Sigh, getting a amateur radio operator license is not difficult, It is if you are trying to run from someone who's liable to pluck your address out of a public database (like the one the FCC runs) and come after you, possibly with murderous intent. It is if the government persists in misgendering you because hating you is politically fashionable in some circles. I'm not arguing that it's conceptually difficult, but…

I haven't done it but I've heard you can use a PO box for your license.

You can in the USA. Mine does.

Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#33

Post by Johnythree »

stefan_ wrote: post #18
squarefoot wrote: post #11

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…

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.

Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#34

Post by Johnythree »

Aloha wrote: post #20
squarefoot wrote: post #11

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…

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.

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.

Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#35

Post by Johnythree »

Spivak wrote: post #15
Joel_Mckay wrote: post #3

Earlier quoted context omitted.

Sigh, getting a amateur radio operator license is not difficult, and teaches people such wonderful concepts like avoiding interference with noisy transmitters. 1. If you are using a modulated square-wave without a filter, than the harmonics/overtones will be splattering the local spectrum. 2. Tools like KerberosSDR/KrakenSDR make triangulating illegal broadcasts rather trivial. 3. Keep in mind, some places have a $5k…

That’s cool and all, but getting an amateur radio license would not have allowed me to do what I want legally anyway which is run a low power music FM station broadcasting to common radios in my own apartment. I do use a filter but it wouldn’t matter if I didn’t, I don’t really care that I’m splattering the spectrum when the broadcast doesn’t even make it through my walls.

> when the broadcast doesn’t even make it through my walls.

But you don't know. If it's radiating a few milliwatts (or even microwatts), that's sufficient signal to be hear across town in a sensitive receiver.

The really sad thing is that the unwanted signals could well be stronger than the unwanted one due to unknown resonances in the antenna wire, etc.

Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#36

Post by Johnythree »

BrandoElFollito wrote: post #7

One needs to be careful with such home made devices as they may clash with the officially assigned frequencies. We had in France a similar story where a village was losing 3G/4G connectivity almost every day around 22:00. It seemed that there are interferences with something. A special car was sent by the national regulation entity, it was trying to triangulate the source. They found a house where the owner bought a…

In my small town, local TV reception was being completely wiped out by interference.

When the Radio Inspector came to check, he found a rechargeable torch sitting in it's charger bracket, which generated horrendous interference whenever the batteries reached full.

Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#37

Post by Johnythree »

cpsns wrote: post #24
natas wrote: post #21

this is completely illegal.

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.

Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#38

Post by Johnythree »

pifm_guy wrote: post #16

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.

Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#39

Post by tb_technical »

esskay wrote: post #26
tb_technical wrote: post #25

Earlier quoted context omitted.

Basically, the FCC can slap you with an expensive fine when the local HAMs rat you out.

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.

Here's a link describing the activity, in case you're interested: https://hamradioplanet.com/what-is-a-fox-hunt-in-ham-radio/

Re: Turn Raspberry Pi’s GPIO into an FM Transmitter (2012)

#40

Post by noasaservice »

pifm_guy wrote: post #16

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…

RPITX is the spiritual successor of your project!

https://github.com/F5OEO/rpitx

And it does work with nearly every RPi.

And unlike only doing FM (97MHz-108MHz), it can emit from an IQ datastream from 10KHz to 1.5GHz.

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