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Comment by lima | Hacker News Reader

> 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.

Don't try this at home!

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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.

Transmitters are not even expensive. A Baofeng is around $20. Some CW kits are $5.

Those cheap Baofengs will spew harmonics too. Quality transmitters do get a bit more expensive than the stuff that mostly-works.

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.[1]

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.

[1] https://www.ecfr.gov/cgi-bin/text-idx?SID=56381cb4f24a491e7d...

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)

Multiples of 1/4 wavelengths, and if built properly 5/8 wave.

... but 5/8 isn't a multiple of 1/4, Or is that not what you meant?

Right. I should have added more detail. Multiples of 1/8 also work.

Now, you can actually make almost any length work with proper tuning circuitry, but the ones mentioned are the most effective.

Also, there's a big difference between random noise and a modulated signal on a specific frequency, with a matching antenna.

One of those is noise, the other one is a transmission.

there is an antenna - just a wire - but that changes the equation a lot relative to what a normal UART does has part of a system that already meets FCC (or other) regulatory standards.

I'm curious about the gain of the antenna and what consideration has been made re trasmit wavelength and the antenna wire length.

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).

Yep - I totally understand that. I wasn't commenting on wires ability to act as antennas in general - I was commenting on the addition of this wire to the UART for the purpose of wireless comms.

That addition is what makes your original point wrong : "Nothing is happening here that doesn't happen already with normal use of the UART"

I doubt the gain of a jumper wire is enough to cause any "damage".

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).

He connected an antenna to it. That's what makes it illegal.

Ugh. It's not by default "illegal", unless it provably violates Part 15.

Has a Part 15 assessment been done yet? Have we analyzed power emission levels to assert that this thing is indeed 'breaking the law'?

Has this device been checked by the FCC for spurious emissions during normal operation? Did it pass, or is it chinese jank uncertifiable garbage?

Edit: C'mon downvoters. If you make a claim that it's "Illegal", you need to have some sort of proof that its the case. And I'm not immediately seeing it. Further research? Sure.

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.

But the rest of what you say is correct.

If it breaches the power limits I think it is illegal - same as if you used a high power wifi access point with a high gain antenna.

He connected a regular wire to it... That's kinda how you use a UART.

The wire is being used as an antenna. When you use UART, you have the other end of the wire terminated with another circuit. (i.e. The energy goes to into the connecting circuit.)

Because the wire ends at an open circuit, the currents on the wire radiate, turning the wire into an antenna.

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.

Sometimes. Plastic DB9 to unshielded RJ12 is pretty common.

> When you use UART, you have the other end of the wire terminated with another circuit. (i.e. The energy goes to into the connecting circuit.)

The other end is likely a MOSFET with an input impedance on the order of 10 GΩ...

And it's nothing new.

Same is possible with Raspberry Pi GPIO:

https://github.com/F5OEO/rpitx

We measured the output of that exact experiment at our local ham radio club and it was horrible, harmonics and noise all over the spectrum.

THIS! -- I was hoping a ham would chime in here :)

For anyone looking to do experiments like this - which are really cool to do - getting the ham technician licenses is pretty easy and very educational!

The harmonics are above the transmission frequency, so a low pass filter is normally all that is needed to clean this up.

There is a project that uses a Raspberry pi to transmit from is GPIO

To use a home built transmitter you need amateur radio license, no matter what.

I got called away while writing this earlier and rushed my last point which is clearly wrong. It is too late to edit.

Instead: 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.

Not true. Low power FM and AM stations are allowed.

https://www.fcc.gov/media/radio/low-power-radio-general-info...

And there's ISM bands as well. However, all of these unlicensed bands have strict limits.

For any of the fun stuff, you need an amateur radio license :-)

> 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.

Only for amateur bands. ISM, 2.4Ghz and 5Ghz along with a few others don't require a license.

From the article, the effective range seems to be around 10 feet.

IIRC you are generally permitted to use whatever frequency you wish within a very limited radius as long as it is for "research purposes"

Edit: this last point is wrong, as mentioned below exemptions seem to apply for certain frequencies with a low powered device https://www.fcc.gov/media/radio/low-power-radio-general-info...

Can you quote the FCC regulations that make this transmitter legal? (I'm assuming American law, the law in other countries obviously vary)

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.

edit: Here's a PDF explainer on Part 15 devices: https://transition.fcc.gov/Bureaus/Engineering_Technology/Do...

See https://www.fcc.gov/media/radio/low-power-radio-general-info...

I don't live in the United States, and I don't give a damn about the FCC.

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).

Not true. See https://www.fcc.gov/media/radio/low-power-radio-general-info...

Doesn't require factory built, does not require license.

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