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...