> This alone is impressive, but [Ted] isn’t done yet. He realized that this method of transmission was generating some strong frequency harmonics which extended far beyond the theoretical maximum 1 MHz frequency of his UART SDR.
i.e., blast noise all over the spectrum. There's a reason why real transmitters are more expensive than a serial port adapter.
This kind of RF pollution is illegal for obvious reasons and if you manage to disrupt something important (emergency frequencies, mobile networks...), it's gonna be expensive.
I don't think transmitters are made artificially expensive to keep out the newbs. As you said, it's illegal.
And if you're a newb (like me) interested enough to try this at home, you might have read to the end of the article where it states:
> DISCLAIMER: It should go without saying that you should never transmit on any frequencies or at any power levels for which you are not authorized by the local governing body, but I guess I just said it anyway.
But he doesn't have any amplification. Nothing is happening here that doesn't happen already with normal use of the UART. If what he is doing here is illegal then the RF pollution from using the UART normally would be too.
It's an "intentional radiator", in radio terms.
It's very easy to build something that blithers all over the RF spectrum by connecting a digital signal to an antenna. Hook a square wave to an antenna and you're going to get lots of harmonics. This is generally considered a bad thing.
In the early days of digital electronics, a TRS-80 Model I home computer and a Milton Bradley Big Trak toy car would both crash if near each other. Strict regulation of RF emissions, and good protection on the input side, is why we can have so many digital devices without everything randomly crashing.
He didn't say this, but I suspect that his antenna was that specific length for rf reasons - wires work best as antennas when their length is some multiple of the wavelength. (ask an rf engineer if you want details, I'm close enough for discussion but all rf engineers will cry when they read that)
I think other commenters have already sufficiently explained this, but just to make it clear: Any wire functions as an antenne, also in a normal circuit where it is connected to something. The length of the wire will cause more radiation from some frequencies, but some designs will have wire length that matches some of the frequencies emitted by the regular communications performed (probably most of them actually - you rarely send random noise over your serial communication channels).
You might well be right on that. "damage" would be good to define in general for these kinds of things. I'm guessing the FCC regs talk about this to some degree.
So, reality is, this is likely very low-power and likely low-impact experiment and likely not cause a lot of issues with nearby devices that are designed to accept all interference and continue to function or fail gracefully.
But IMHO, it is a good idea for anyone doing these kinds of experiments to think a little bit about RF design and the impacts devices might have ..hence my other comment about the value of HAM technicians license & training (which is fun and pretty easy).
This is mostly correct, except for FCC exceptions for stuff you build yourself. For example, it is legal to broadcast on the FM band if your signal meets certain signal strength criteria. Also, the FCC itself doesn't do assessments, they are done by independent (expensive) labs.
Wires connected to a closed circuit will radiate too, if you drive them at high frequency. Maybe not efficiently, but it'll still radiate. There's nothing special about an open circuit that makes an antenna work.
Shielding (e.g. coax, or a properly grounded enclosure) is what prevents ordinary wires from becoming antennas.
Normal rs232 is run through shielded cables as well. It was standard practice to wire the d-sub connectors to the cable shields. The wires themselves may not have been shielded, but the cable was shielded against being an antenna.
I got called away while writing this earlier and rushed my last point which is clearly wrong. It is too late to edit.
To use a home built transmitter you need amateur radio license, except in certain specific low power situations. To comply with those situations would be difficult without specialised test gear.
> However, all of these unlicensed bands have strict limits.
This is absolutely key here - even in unlicensed bands one cannot just start using the spectrum without careful consideration of regulations for that spectrum.
When I was working in the 902-928 Mhz (FCC and similar) and 860s (for ETSI) for RFID we had to do a lot of work to ensure our transmit power, power ramp up, frequency hopping and listen before talk were all implemented properly and demonstrable for certification. It was not trivial.
The FCC regulations are very long, hard to understand, and occasionally contradictory - I know, I work in the industry. The closest section of the law that comes to mind is for very low power (typically under 1mW) devices that do not require the user to have a licence. These are so called "part 15" devices, which refers to the Code of Federal Regulations (CFR) Title 47, Part 15.
While the user of these sorts of devices doesn't require a licence, the device itself needs to be certified, so that, among other things, it doesn't spew noise all over the RF spectrum. This device wouldn't pass.
There are also experimental licences that can be held by businesses such as RF equipment manufacturers that allow for using equipment that hasn't yet received FCC approval. They are usually band restricted, geographically restricted and power restricted, and you must take care to not have high levels of spurious emissions.
You should - many regulators around the world are modeled after FCC. When I was doing regulatory test work for RFID readers (which were transmitting at 1W ) IIRC Canada, Korea and Japan were very similar to FCC.
Additionally - if you are in Europe - regulations are defined by a body called ETSI (https://www.etsi.org/). For RFID these regs were quite different from FCC - we had to do listen before talk and a variety of other things to ensure compliance.
You can see from gqrx that his rtl-sdr can barely pick this up when it's 6 inches away on his desk. Somehow I don't think this is causing a significant amount of harmful interference. (Admittedly it is quite a distant harmonic - the actual signal will of course be stronger).
Actually, this is a class D transmitter. Minus the usual output filter to eliminate the higher harmonics. Look at that spectrum chart. The second and third harmonics are huge!
It's quite possible to build something that will turn a square wave into a clean sine wave, and you need that. You can also generate waveforms digitally that have weaker higher harmonics, which means the output filter is simpler.
There's something called a class E amplifier, which is like a class D but with an unusual analog tank circuit on the output end to clean things up.
A device like this with no output filter is an annoyance, not a communications device.
The SDR they talk about in article introduction is specifically using cheap USB TV tuners in combination with open source software to capture various signals. The list of applications is really impressive: https://www.rtl-sdr.com/tag/applications-2/
If you want to filter these signals, and are a ham, I use these on my Raspberry Pi https://www.qrp-labs.com/lpfkit.html and have been heard all around the US daily, and also across both oceans a few times without an amp.
As mentioned in the first sentence, the RTL-SDR project is out there. You can easily buy a $20 USB Software Defined Radio with software that will handle everything for you and lets you play around with an SDR: https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-dongles/
An interesting thing from reading about practical experiments in EM eavesdropping is that it's not just the obvious emanations that you have to worry about. Your sidechannels have sidechannels. In that paper, for example, the actual communication radiates and can be received in the shortwave band, but also a frequency-modulated version of it appears in the FM radio band. Presumably a bit of power-supply droop is modulating a local oscillator causing it to transmit a clear signal.
Most keyboards don't have an antenna attached. While you can keylog a keyboard via emissions like this, you have to have physical access to the office to do so - at that point putting a usb keylogger inline with the keyboard is easier and more reliable. (or substitution your own keyboard with a real radio)
Not sure what is inside of one of those keyboard adapters, but this is something called a serial adapter (or, more precisely, UART adapter). It is usually used to program microcontrollers (read Arduinos), so everyone dabbling with those (typical audience of hackaday.com) probably has at least one at home, they cost like $2-3.