Pi Zero FM Radio

Similar to my story about the SanDisk Connect device, this idea started it’s roots in my trip to DefCon 2018, specifically my experience in the Wireless CTF. I wanted to recreate some of the RF signals I was able to “hear” using my gear. I could hear quite a bit but had no idea what to do with it. There were quite a few people running the CTF, they were all over the room. The issue was my pride not simply wanting to reach out and admit what I am: a pleb.

So when coming back to my desk at home, I took my notes and started to research methods to transmit the similar items that I had heard and what was covered on the debrief. My HackRF was still on the way from Amazon but I was eager to get to work. I believe I initially started researching Raspberry Pirate Radio as found here. This seemed a little more than I was going to start with since I really hadn’t played with any of the Raspberry Pi models at that point. In a flurry of googling and reading, I landed on this article, also by Makezine.

I didn’t make a throw-able as I wasn’t going to simply do as the article suggested and take a bare Pi Zero and attach a battery with some magnets and toss it up somewhere. What I wanted to do is transmit in the FM band on the cheap at an extremely short distance. I couldn’t pinpoint the exact stance of the FCC. A website stated the FCC wants a FCC ID on any transmitter, such as one you might pickup to broadcast your smartphone, or back the day, a diskman to your automobile radio. Reading §15.239 of Part 15 of subchapter A of Chapter A of Title 47 wasn’t too clear.

Here’s what I did understand as a licensed ham, it’s my responsibility to understand what I was doing and if I start interfering with normal FM transmissions as well as impacting users around me, to knock it off. So I gathered the materials at my local Microcenter and came home to get this party started.

I started the project just as described in the article.

1. Solder the wire antenna.
2. Download and Install Raspbian.
3. Boot the Pi Zero.
4. Install PiFM and MPG123.
5. Run PiFM.

The install of PiFM and MPG123 is straight forward for someone with experience downloading and installing from either apt or cloning from github.

MPG123:

sudo apt install mpg123

PiFM:

git clone https://github.com/rm-hull/pifm

To run pifm with the mp3 of my choice, execute the following in the terminal:

sudo /usr/bin/mpg123 -4 -s -Z /home/pi/Music/lincoln.mp3 | sudo /home/pi/PiFM/pifm/pifm - 88.5

Now the lincoln.mp3 is an interesting thing. Shortwave is something I always have found fascinating but haven’t explored. On the hunt for some mp3 files for some number stations, I came across http://priyom.org as well as http://users.telenet.be/d.rijmenants/. Just a short wget away from downloading the file to my Pi.

Oddly enough my selection of FM radios around the house is pretty slim. I wasn’t about to pull the radio of the shelf in the garage to listen to the transmission in the house. Since I have a few RTL-SDR dongles, why not use them. The surprising finding was the transmission or spurious emission that the Pi Zero didn’t stop with a ctrl-c. I had to restart the blasted thing to get the Pi to stop.

PiFM continues to transmit dead air.

This was concerning. I wanted to tinker and transmit but also I’m trying to operate the the realm of being a responsible adult. Although if you are transmitting and then shut it off, then the impact might not be as great. I wasn’t too keen on the idea that you stop the program but the Pi continued to transmit dead air. I went back to researching aka google. I came across this github: https://github.com/markondej/fm_transmitter. Taking a look at the readme, I knew this was well within my wheelhouse to give a go.

mkdir FM_Transmitter
git clone https://github.com/markondej/fm_transmitter

Following the directions and some testing I resulted in this command:

sudo fm_transmitter/fm_transmitter -f 88.5 -r fm_transmitter/star_wars.wav

I mean it was totally worth a shot when there’s a star wars wav file in the repository. Sure enough, on the FFT plot and waterfall of gqrx, the wav file was being transmitted. When I used ctrl-c to stop, the transmission stopped.

The one thing I noticed was the limitation to wav files. Not that I did much research beyond getting the fm_transmitter to just play the default star wars file. I started to think about another idea I had. If I wanted to transmit Morse code, since I could hear it but not decode it in the desert, I had already found a website that will pump out an mp3 of Morse code. Back to googling.

What I came up with was the following software and process to convert a mp3 file to a wav file on the Pi Zero. Install ffmpeg and sox. Then using those, execute the following on the mp3 of your choosing:

ffmpeg -i lincoln.mp3 lincoln.wav
sox --ignore-length lincoln.wav lincoln2.wav

There’s an error which I can’t recall exactly that when you take the file directly from the ffmpeg conversion to fm_transmitter that it complains about the file being 0 length or empty. Using sox will tidy things up.

Transmitting using FM_Transmitter.

So was it worth it? It depends. Relatively inexpensive compared to a regular Pi. I forget what I paid for the case and the SD card but the Pi Zero W was $5. A little experimentation is good for the tinkerer. It was interesting to hook up different RTL-SDR dongles and listen. It’s not a clear signal on any RTL-SDR dongle that I have. Even the Baofeng can’t get it to come in clearly. There goes my idea of having a Pi Zero as a Morse code transmitter. The trip was worth the cost though. A opportunity to learn something new. Research how to turn mp3s into wav files. I did use the Pi Zero using a camera to broadcast the image out the internet.

The Pi Zero W with hole in the lid for the wire.

Thank you for reading, hopefully you’ve learned something. Twitter: @grnbeltwarrior Github: https://github.com/grnbeltwarrior