Wait, an antenna connected to the GPIO pin on a RaspberryPi for transmissions? Correct me if I'm wrong, but doesn't the Pi output square waves, that would cause harmonics if transmitted?
There was an old RPi project released a while ago (PiRadio I think?) which simply involved attaching a jump lead to GPIO pin 4 which allowed the transmission of FM radio signals.
It worked on the principal that the Pi had a stupidly powerful clock and thus could be used as a transmitter.
I did a little experiment where I hooked it up to a yagi antenna and asked my dad to set his cars radio to the frequency I set the software to transmit on and drive around the block to test the distance.
Not only did it work well but I think I heard someone listening to the radio in their home near mine open their front door and yell out "who ever is doing that could you please stop it!" as it was interfering/jamming what they were listening to.
I'm definitely checking this project out though, thanks for posting.
EDIT: That link was definitely part of what I was researching at the time (as it is linked in the project article I posted and I remember reading about the limitations regarding stereo output, as well as the line "Most radio receivers want a signal to be an odd multiple of 0.1 MHz to work properly").
For dramatic effect...during the experiment I actually did make a windows text-to-voice recording saying something along the lines of "you are listening to <my street name> pirate radio" between the stoner metal songs I had lined up (I was listening to lots of QOTSA at the time to be honest).
Yes they should just because they can, it's easier, you don't need extra hardware when an RPi is already there and it's good to know how to do that when no SDR is around.
"But The higher the $$$, the harder it is for people to build one. That’s another allure why I like $150. It’s attainable, and many people already have some parts."
It's this kind of pragmatism that's going to make these affordable tools proliferate and constantly pour cold water over the tech companies Privacy-In-Can™ products and services.
You can buy an SDR that'll output reasonably clean RF for about $150 - an Adalm Pluto, a LimeSDR Mini or a HackRF One clone.
The barrier to entry to radio has never been lower, which is very much a mixed bag. A lot of people are doing really stupid, really illegal stuff with cheap radio hardware. Quite aside from the legalities, it's just bad manners to splatter all over the RF spectrum. Listen to the bands, get yourself an amateur license, learn to build a lowpass filter, then press the transmit button.
>The barrier to entry to radio has never been lower, which is very much a mixed bag. A lot of people are doing really stupid, really illegal stuff with cheap radio hardware. Quite aside from the legalities, it's just bad manners to splatter all over the RF spectrum.
I get this, but despite it all, I'm going to be more interested at the end of the day, in principle, in solutions that would involve low cost antenna design/filtration techniques/etc, that make the cheap stuff better compared to the buy one component that's 2x amount of an entire system kind of solutions.
Because the reality is that this genie isn't going to go back into the bottle, legalities aside or not.
It's not RF snobbery, the point is that this approach to transmitting RF signal is shitting all over the spectrum with harmonics and potentially interfering with other legitimate activity.
Sure, you can get shielding and bandpass filters, but by then you're back at $100 bucks and have terrible radio to boot. The Pluto and Lime Mini are very capable transceivers for ~$150.
The RPi isn't going to bring down airliners and start WWIII, but radio spectrum isn't going to work if people don't cooperate and be good neighbors.
You need a band-filter. In the simplest case, the antenna is one. I haven't looked into it in more detail, but I would guess that they exploit the higher harmonics of the square wave to actually reach the 1.5GHz.
Yes, the Pi only has digital pins. I didn't really watch the video but I assume any antenna is connected to another board and that board is controlled through the Pi pins.
Actually, looking at the photo of the internals, it looks like the only thing connected to GPIO is the GPS board, all the radio hardware is connected to USB ports.
Turns out, after reviewing the interference from a GPIO pin, is actually very low (about 5% non-target frequency). I do understand I still have to deal with a comb filter output, but is minuscule from my inspections.
Im working on building a programmable filter network with independent low and high bandpass filters. But nothing available for the amateur market is sufficient. There's some stuff for the Ka and Ku bands (high microwave bands 12GHz-40GHz) but nothing for 50MHz-1200MHz. Nobody except for SigInt really cares about such a broad set of frequencies.
Thats why I need to make a programmable filter network...
The antennas for Tx/Rx are rated for 50MHz-1200MHz. The Rtlsdr is capable of 24MHz-1.7GHz, and the Rpitx is capable of 100KHz-1.5GHz.
Using a generic bandpass from 50MHz-1200MHz would allow combing down the powers of said frequency. Whereas a proper filter network could selectively notch out a few MHz window to prevent spurious transmissions. That device would also be very useful for things like the HackRF, PLUTO, and LimeSDR. As much as they claim they generate clean signals, they still have problems with harmonics, sometimes at weird powers (as in fractional exponents, not wattage).
