4 ms·

As an amateur radio operator, I think this is fascinating, because they claim it can transmit on the 2m VHF band (144-148MHz) that's popular among hams. But I

As an amateur radio operator, I think this is fascinating, because they claim it can transmit on the 2m VHF band (144-148MHz) that's popular among hams. But I have some concerns about its signal characteristics, since you're essentially abusing digital hardware (GPIO) to produce a modulated output it wasn't designed to produce. Someone with a spectrum analyzer should check to see if it produces any nasty harmonics, or similar noise, that could interfere with other stations.

sbierwagen 13y ago
It transmits ~10mW, though.
fhars 13y ago
If you need a datapoint on how noisy those signals are: some people have combined a 144.6Mhz Raspberry Pi output with a narrow passive bandpass filter to generate a relatively clean and powerful 433MHz signal (the third harmonic, which is quite strong in a square wave): http://www.skagmo.com/page.php?p=projects/22_pihat http://www.skagmo.com/page.php?p=projects/22_pihat
- a-priori 13y ago
  Yes, assuming there's strong harmonics (I'd be real surprised if there weren't), you should definitely put a bandpass filter between the Raspberry Pi and the antenna like these guys did. Thanks for the link! That's a clever way of using harmonics to your advantage.
- aortega 13y ago
  Recently I did a similar project in which a malicious hardware device leaks a victim's data (Password, credit card #, etc.) through a similar technique. We used an FPGA though, so with a 100 Mhz fundamental frequency we had harmonics well into the 1.5 Ghz. We called it CPU-SDR. Video here: https://sites.google.com/a/groundworkstech.com/index/blog/rsa-2014---about-cpu-backdoors-and-data-exfiltration https://sites.google.com/a/groundworkstech.com/index/blog/rs... Bonus: Using mutiple PCB traces, you have a basic Yagi-like directional antenna. Feeding different traces you can modify the RF radiation pattern it in real-time.
  - jrockway 13y ago
    You've indeed invented MIMO.
    - aortega 13y ago
      It's more like a Yagi, as currently I feed only one of the "antennas" and the modulation is very limited. But messing with the phases should be possible.
Florin_Andrei 13y ago
Fellow HAM here. I don't doubt that the spectrum coming out of that thing is pretty horrific. There are no filters, and who knows what the wave form is (rectangular?).
- keithpeter 13y ago
  Hang a bandpass LC filter on the output then feed to a dipole? Disclaimer: not an active HAM, used to clean seagull crap off 500KHz distress aerials on small ships in the last century.
- aortega 13y ago
  I don't think a rectangular wave can propagate through the ether :) Output from the digital pin is very rectangular. But you have many sinusoidal‎ harmonics ready to be fired into the air if you plug a small antenna on it.
  - lutusp 13y ago
    > I don't think a rectangular wave can propagate through the ether ... Of course it can. A square wave is just a series of discrete sine components, each of which will happily propagate through the air. If this were not true, WiFi wouldn't work -- it also relies on multiple, coordinated frequencies and complex modulation.
    - a-priori 13y ago
      Right, in ideal hardware. The problem is that in order to transmit an ideal square wave, you need infinite bandwidth. The problem being that any real circuit has a finite band it operates on. It will filter the components of the wave that are outside of its bandwidth, leading to 'ringing' artifacts (Gibbs phenomenon).
      - lutusp 13y ago
        > Right, in ideal hardware. The problem is that in order to transmit an ideal square wave, you need infinite bandwidth. On the contrary. An ideal square wave's higher harmonics decline rather quickly: http://en.wikipedia.org/wiki/Square_wave#Examining_the_square_wave http://en.wikipedia.org/wiki/Square_wave#Examining_the_squar... In other words, there's enough energy to cause problems at 2, 6, 10 ... times f, but at rapidly declining levels. > It will filter the components of the wave that are outside of its bandwidth, leading to 'ringing' artifacts ... Ringing artifacts are present in a system whose bandwidth rolls off abruptly. It's less significant in a system with a gradual drop-off in response, like a piece of wire acting as a broadband antenna.
bananas 13y ago
Doubt it's got much range. I'd be surprised if it goes 50ft. Most of the power is going to be lost unless you have a decent antenna on the end and something to decouple it from the host circuit. Bandpass wouldn't go amiss though.
- th0ma5 13y ago
  I was able to pick up an indoor transmission from my car up to about 1000 feet away. However after losing the signal the car radio had to be within 400 feet before it locked on again.
  - bananas 13y ago
    Ouch that's pretty huge.
fnordfnordfnord 13y ago
Yeah, it's nasty, but also gutless.
ja27 13y ago
The winner of Make's 2014 Raspi contest does SSTV from a Pi: http://www.instructables.com/id/Raspberry-Pi-security-Slow-Scan-Television-Camera/?ALLSTEPS http://www.instructables.com/id/Raspberry-Pi-security-Slow-S...
th0ma5 13y ago
I used an RTLSDR to look at the spectrum. If you're plugged into the wall, you get ripple. Also the harmonics are pretty loud too. The WSPR example above shows the filtering they put in. In theory at low power like someone else said it isn't ideal, but wouldn't upset too many people so long as you didn't amplify before filtering.

sbierwagen 13y ago

It transmits ~10mW, though.

fhars 13y ago

If you need a datapoint on how noisy those signals are: some people have combined a 144.6Mhz Raspberry Pi output with a narrow passive bandpass filter to generate a relatively clean and powerful 433MHz signal (the third harmonic, which is quite strong in a square wave): http://www.skagmo.com/page.php?p=projects/22_pihat http://www.skagmo.com/page.php?p=projects/22_pihat

a-priori 13y ago
Yes, assuming there's strong harmonics (I'd be real surprised if there weren't), you should definitely put a bandpass filter between the Raspberry Pi and the antenna like these guys did. Thanks for the link! That's a clever way of using harmonics to your advantage.
aortega 13y ago
Recently I did a similar project in which a malicious hardware device leaks a victim's data (Password, credit card #, etc.) through a similar technique. We used an FPGA though, so with a 100 Mhz fundamental frequency we had harmonics well into the 1.5 Ghz. We called it CPU-SDR. Video here: https://sites.google.com/a/groundworkstech.com/index/blog/rsa-2014---about-cpu-backdoors-and-data-exfiltration https://sites.google.com/a/groundworkstech.com/index/blog/rs... Bonus: Using mutiple PCB traces, you have a basic Yagi-like directional antenna. Feeding different traces you can modify the RF radiation pattern it in real-time.
- jrockway 13y ago
  You've indeed invented MIMO.
  - aortega 13y ago
    It's more like a Yagi, as currently I feed only one of the "antennas" and the modulation is very limited. But messing with the phases should be possible.

Florin_Andrei 13y ago

Fellow HAM here. I don't doubt that the spectrum coming out of that thing is pretty horrific. There are no filters, and who knows what the wave form is (rectangular?).

keithpeter 13y ago
Hang a bandpass LC filter on the output then feed to a dipole? Disclaimer: not an active HAM, used to clean seagull crap off 500KHz distress aerials on small ships in the last century.
aortega 13y ago
I don't think a rectangular wave can propagate through the ether :) Output from the digital pin is very rectangular. But you have many sinusoidal‎ harmonics ready to be fired into the air if you plug a small antenna on it.
- lutusp 13y ago
  > I don't think a rectangular wave can propagate through the ether ... Of course it can. A square wave is just a series of discrete sine components, each of which will happily propagate through the air. If this were not true, WiFi wouldn't work -- it also relies on multiple, coordinated frequencies and complex modulation.
  - a-priori 13y ago
    Right, in ideal hardware. The problem is that in order to transmit an ideal square wave, you need infinite bandwidth. The problem being that any real circuit has a finite band it operates on. It will filter the components of the wave that are outside of its bandwidth, leading to 'ringing' artifacts (Gibbs phenomenon).
    - lutusp 13y ago
      > Right, in ideal hardware. The problem is that in order to transmit an ideal square wave, you need infinite bandwidth. On the contrary. An ideal square wave's higher harmonics decline rather quickly: http://en.wikipedia.org/wiki/Square_wave#Examining_the_square_wave http://en.wikipedia.org/wiki/Square_wave#Examining_the_squar... In other words, there's enough energy to cause problems at 2, 6, 10 ... times f, but at rapidly declining levels. > It will filter the components of the wave that are outside of its bandwidth, leading to 'ringing' artifacts ... Ringing artifacts are present in a system whose bandwidth rolls off abruptly. It's less significant in a system with a gradual drop-off in response, like a piece of wire acting as a broadband antenna.

bananas 13y ago

Doubt it's got much range. I'd be surprised if it goes 50ft. Most of the power is going to be lost unless you have a decent antenna on the end and something to decouple it from the host circuit. Bandpass wouldn't go amiss though.

th0ma5 13y ago
I was able to pick up an indoor transmission from my car up to about 1000 feet away. However after losing the signal the car radio had to be within 400 feet before it locked on again.
- bananas 13y ago
  Ouch that's pretty huge.

fnordfnordfnord 13y ago

Yeah, it's nasty, but also gutless.

ja27 13y ago

The winner of Make's 2014 Raspi contest does SSTV from a Pi: http://www.instructables.com/id/Raspberry-Pi-security-Slow-Scan-Television-Camera/?ALLSTEPS http://www.instructables.com/id/Raspberry-Pi-security-Slow-S...

th0ma5 13y ago

I used an RTLSDR to look at the spectrum. If you're plugged into the wall, you get ripple. Also the harmonics are pretty loud too. The WSPR example above shows the filtering they put in. In theory at low power like someone else said it isn't ideal, but wouldn't upset too many people so long as you didn't amplify before filtering.