
Serial Port SDR - walterbell
https://hackaday.com/2018/12/06/your-usb-serial-adapter-just-became-a-sdr/
======
lima
> _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!

~~~
poizan42
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.

~~~
bluGill
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)

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

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

~~~
wglb
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.

------
cheschire
Not being deeply into signals geekery, I had to lookup SDR. I believe the
article is referring to a Software-Defined Radio.

[https://en.wikipedia.org/wiki/Software-
defined_radio](https://en.wikipedia.org/wiki/Software-defined_radio)

~~~
codeulike
Thanks.

~~~
veli_joza
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/](https://www.rtl-sdr.com/tag/applications-2/)

------
sorenjan
Reminds me of using an output pin on Raspberry Pi to transmit FM radio:
[http://icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry...](http://icrobotics.co.uk/wiki/index.php/Turning_the_Raspberry_Pi_Into_an_FM_Transmitter)

~~~
rlonstein
Reminds me of abusing the CRT on the TRS-80 to play "music" on an AM radio.
Except we were kids and had very little idea what we were doing.

~~~
dogecoinbase
Tempest for Eliza is an cool old piece of code to play mp3s over your CRT via
AM: [http://www.erikyyy.de/tempest/](http://www.erikyyy.de/tempest/)

------
th0ma5
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](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.

------
BuildTheRobots
USB serial dongle? pah, use the system-bus on the motherboard ;)
[https://github.com/fulldecent/system-bus-
radio](https://github.com/fulldecent/system-bus-radio)

~~~
devereaux
Now that is rad!

------
krenzo
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/](https://www.rtl-sdr.com/buy-rtl-sdr-dvb-t-
dongles/)

~~~
squarefoot
For roughly the same price there are some DVR boards that could be converted
to SDR operation once they're hacked. They already use embedded Linux although
no source is provided.

[https://www.aliexpress.com/item/OUERTECH-5-in-1-AHD-CVI-
TVI-...](https://www.aliexpress.com/item/OUERTECH-5-in-1-AHD-CVI-TVI-IP-
CVBS-4CH-CCTV-DVR-board-1080N-2-SATA/32860968547.html)

~~~
rhinoceraptor
You can also get HackRF clones, for about $100:

[https://www.aliexpress.com/item/HackRF-One-1MHz-to-6GHz-
SDR-...](https://www.aliexpress.com/item/HackRF-One-1MHz-to-6GHz-SDR-Platform-
Software-Defined-Radio-Development-Board/32809842995.html)

~~~
squarefoot
Yup, I know the HackRF. Being too old to write Santa a letter I'm already
saving money for that gift:).

Anyway my point was that those cheapo DVR boards are just asking to be
repurposed for other tasks, SDR is just one of them, that chipset specs are
interesting.
[https://www.burglaryalarmsystem.com/pdf/Hi3520.pdf](https://www.burglaryalarmsystem.com/pdf/Hi3520.pdf)

Here are some firmware files for a few models, hopefully compatible with that
one. Binwalk-ing them shows they're normal ARM Linux images.

[https://domar.com/support/softwares-firmwares/ip-firmware-
up...](https://domar.com/support/softwares-firmwares/ip-firmware-updates.html)

------
heywire
Sounds like an interesting approach to data exfiltration on an air-gapped
system, provided the right pieces are in place.

------
apo
It sounds interesting. Beyond controlling RC vehicles, what would be some
applications of software defined radios (SDRs)?

By the way, it looks like a USB-VGA dongle can be manipulated in a similar
way:

[https://news.ycombinator.com/item?id=16907260](https://news.ycombinator.com/item?id=16907260)

~~~
dfox
We used to half joke that graphics card is in fact nothing more than very good
and cheap DDS signal generator.

And as for SDR applications: most of modern radio devices use software defined
radios and many modern radio interfaces cannot be reasonably implemented in
any other way.

------
IIAOPSW
Well now we know what radio waves are produced by 0x55. How about other
characters? I'm willing to bet that everyone's wired keyboard is an SDR so to
speak and a well placed receiver can keylog you.

~~~
bluGill
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)

~~~
IIAOPSW
>Most keyboards don't have an antenna attached.

What about meter long wire that runs from the keyboard to the back of the
tower.

~~~
bluGill
it is twisted pair. Not great shielding, but enough to make the signal weak.

------
hexagon5un
This is so rad. It's like Tempest, but on purpose. And I love that he hammers
it down by filtering out the fundamental and transmitting over 10 feet.

------
zoobab
RF hackers should have their own country:

[http://www.zoobab.com/zooland](http://www.zoobab.com/zooland)

"Zooland is a new country where there are no regulations on electromagnetic
transmissions.

Those regulations are from the past and stifle innovation and development in
RF technologies.

They also encourage expensive communications.

Current RF regulations are a pure spacial and economic waste."

------
ngcc_hk
Still remember doing some programming on ICl terminal and a colleague bought a
radio controled car for fun. Then we found out these car will be taken over by
the terminal.

Not very impressed then as we are sitting there nights and days.

------
StillBored
This is just the RF version of the apple ][ speaker playing audio. Toggle a
binary pin attached to something analog fast enough and you can approximate a
signal (plus a ton of harmonics).

------
rsp1984
Can someone with understanding of the matter ELI5 why this matters?

~~~
Fordec
Without having to hack and/or expose I/O pins, anything with a USB port that
can run the open source code can be given wireless communication capabilities
with a very common and cheap chip

~~~
sokoloff
With the tradeoff of being incredibly disruptive to the RF spectrum (and
therefore strongly ill-advised for use in a product)

------
emilfihlman
How is this different from an AVR reading values from uart and banging ports?

It's not.

------
vtesucks
I have a converter that I plug into my USB socket and then I can plug my old
style keyboard into this converter. Is that what is being talked about here?
The picture looks different.

~~~
jablan
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.

