Have you ever wondered what's in the radio waves zipping invisibly around you every day?  Software-defined radio (SDR) is a great tool to explore radio signals using a computer and inexpensive radio tuner. With SDR you can examine many radio signals such as FM radio, television, emergency & weather radio, citizen band (CB), wireless protocols and much more.

Although dedicated SDR hardware like the HackRF allow you to tune an immense range of the radio spectrum, you can easily get started with SDR using a Raspberry Pi and inexpensive RTL-SDR tuner.  Inspired by the HackRF PortaPack, this project will show you how to build a small portable SDR scanner using a Raspberry Pi, PiTFT, and RTL-SDR radio dongle.  With the Raspberry Pi Freq Show RTL-SDR scanner you can visualize the invisible world of radio!

UNDERSTAND: FREQSHOW DOES NOT PLAY AUDIO. It graphs the frequency and amplitude of RF signals, which can be useful for troubleshooting and developing a basic understanding of radio and wireless protocols, but it does not decode nor demodulate them.

Before you get started it will help to familiarize yourself with a few other guides for more background information:

Also for some inspiration on what you can do with SDR, check out these excellent presentations from previous DEF CON conferences:

Before using SDR and scanning tools be sure to check the laws for your country. In some countries, like the US, there are frequencies for cell phones and other communication that you cannot legally tune: http://en.wikipedia.org/wiki/Scanner_(radio)#Legislation

To build this project you'll need the following hardware:

  • Raspberry Pi computer. Most any model will do. This project uses the GPIO header and one USB-A port…so anything but Compute Module or headerless Zero units can work.
  • RTL-SDR software radio USB dongle
  • 3.5" PiTFT display
    • The software is designed to run on a 480x320 3.5" PiTFT display. If you want to get your hands dirty in the code it should be possible to port it to smaller displays like the 320x240 2.8" PiTFT by changing font sizes and other dimensions. But as-is, use the larger 480x320 screen.
    • Many users have reported success using the official 7" Raspberry Pi Touch Display instead of the PiTFT. We’ll provide alternate steps to try.

You might also consider some extra parts to extend the project:

Initial System Setup

Using Raspberry Pi Imager, set up a SD card with the current Raspberry Pi Bullseye 32-bit Desktop operating system. Steer clear of the “Lite” and “64-bit” versions…32-bit Desktop is what’s needed here. It’s recommended to set up system credentials and WiFi network (including country) ahead of time using the Advanced Options dialog (gear icon). Enable SSH if you’ll be logging in remotely over a network. Then write the card, move it to the Pi, and allow a minute or two for first boot.

Follow the 3.5" PiTFT guide to learn how to install and use the PiTFT (skip this part if using the official 7" Raspberry Pi Touch Display)…

  • You can disregard what it says about specific operating system versions, as we established above that this works with the current “Bullseye” release.
  • Download the installer script as shown there.
  • Scroll down to the “FBCP Install Commands” section (NOT the Console Mode install) and use the PiTFT 3.5" Resistive touchscreen setup command. After this runs, you’ll be prompted to reboot the system…that’s not strictly necessary yet, you can proceed with more installation below…but there’s no harm in rebooting here if you just want to test that the screen is working.
  • Skip ahead in the PiTFT guide to the “PiTFT PyGame Tips” page. Ignore the install steps, just scroll down to the “Running SDL 1.2” section and follow the steps there. This is required to make the PiTFT touchscreen work!

Finally plug in the RTL-SDR dongle to a USB port on the Raspberry Pi.  

That's it, your hardware is ready to go!

Dependencies

Before running the software you'll need to install a few depdendencies.  Make sure you're running the Raspbian operating system and it has been configured to work with the PiTFT as explained above.  You will also need to make sure the Pi has access to the internet through a wired or wireless networking connection.

Connect to a terminal on the Pi and execute the following commands to install the dependencies:

sudo apt-get update
sudo apt-get install git cmake build-essential python3-pip libusb-1.0-0-dev libsdl2-2.0-0 libatlas-base-dev librtlsdr0 pandoc
sudo pip3 install numpy pygame pyrtlsdr

Don't worry if you see warnings that certain packages are already installed, you can ignore those and move on.

At this point all the dependencies should be installed and you're ready to download and install the project software.

Installation

Installing the software for this project is as easy as downloading its Python source code.  At a terminal on the Pi run the following commands to download the source:

cd ~
git clone https://github.com/adafruit/FreqShow.git
cd FreqShow

If using the official 7" Raspberry Pi Touchscreen instead of a PiTFT, edit the file freqshow.py and comment out the following lines as shown (starting around line 73). Do NOT do this if using a PiTFT!

#os.putenv('SDL_VIDEODRIVER', 'fbcon')
#os.putenv('SDL_FBDEV'      , '/dev/fb1')
#os.putenv('SDL_MOUSEDRV'   , 'TSLIB')
#os.putenv('SDL_MOUSEDEV'   , '/dev/input/touchscreen')

That's it, at this point the software should be downloaded and ready to run!  Continue on to the next page to learn how to run the software.

To start the program connect to a terminal session on the Raspberry Pi and navigate to the FreqShow folder (if you aren't there already), then run:

sudo python freqshow.py

After a few moments a splash screen should display on the PiTFT as the program loads:

If the program fails to load go back to the installation steps and carefully check all the dependencies are installed, then try again.

Note that if you see an error like "Kernel driver is active, or device is claimed by second instance..." it could mean the TV tuner driver is still loaded by the kernel.  Normally when the RTL-SDR code is compiled with the steps in this guide it should automatically unload the conflicting kernel driver, but if that fails you can manually stop the conflicting module from loading.  See the "Avoid the Raspberry Pi to load a kernel module" step from this blog post to see what kernel modules should be added to the /etc/modprobe.d/raspi-blacklist.conf file (make sure to reboot the Pi after editing that file).

Once the program loads it will start by displaying an animated frequncy graph, and menu of options.  The graph displays the intensity of radio signals (measured in decibels) across a range of frequencies.  The taller a peak on the graph, the higher the intensity of the signal at that frequency.

Below is an overview of the important parts of the graph:

To the left of the graph the minimum and maximum intensity values are displayed in decibels.  The top value is the maximum intensity, in this case 50 dB, which means a point at the very top of the graph has an intensity of 50 dB.  Similarly the bottom value is the minimum intensity, -3 dB, so points at the bottom of the graph have an intensity of -3 dB.  Points between the top and bottom of the graph have an intensity in between the minimum and maximum, so for example a point in the middle of the graph would have an intensity around 26-27 dB.

At the bottom of the graph the range of frequencies are displayed.  The minimum frequency is displayed at the bottom left, the center frequency in the middle, and the maximum frequency at the bottom right.  In the image above you can see a total range of about 2 MHz of frequencies centered at 90.3 MHz.

The frequency 90.3 MHz actually corresponds to an FM radio station (KEXP in Seattle, WA).  It's interesting to see the spikes of intensity near the center frequency--the graph is visualizing what an FM radio plays as audio!

REMINDER: FreqShow only DISPLAYS THE SPECTRUM and DOES NOT decode or demodulate the radio signal into audio or data!

You can change the center frequency and other settings by pressing the CONFIG button in the upper left corner.  A list of settings such as the following should appear:

Press the CENTER FREQ button at the top to bring up a dialog to change the center frequency:

You can change the center frequency just like inputting numbers in a calculator.  Press the CLEAR button to completely erase the current frequency value, enter new digits by pressing numbers (or the decimal point), and delete the last digit by pressing DELETE.  

Once you've entered a new value you can press ACCEPT to accept it and change the center frequency.  If you don't want to change the frequency press CANCEL to dismiss the dialog without changing the value.

Try changing the center frequency to that of an FM radio station in your area.  FM radio in the US is broadcast over 88 to 108 Mhz so pick a value within that range.  After accepting a new center frequency you should be returned to the settings list.  Press BACK in the lower left to return to the frequency graph.  Notice how the graph changes to show the intensity of the signal at the new center frequency.

Another useful way to visualize radio frequency data is with a spectrogram that shows the intensity of the signal over time.  In the main graph view press the SWITCH MODE button at the top of the screen.  You should see the graph change to start building a waterfall plot that scrolls up from the bottom to the top of the screen.  

For example this is a waterfall plot that has entirely filled the screen:

The waterfall plot displays intensity over time.  Each row of the plot represents a point in time, and each pixel color in a row represents the intensity of the signal at that frequency.  Pure blue pixels are the minimum intensity, pure red pixels are the maximum intensities, and a gradient of blue-cyan-yellow-red represents in between intensities.

Notice in the picture above the highest intensity frequencies in the center are red, while the lowest intensity frequencies at the edges are blue.  Smaller peaks are cyan and yellow.

About once a second the graph will scroll up and a new measurement row will be added at the bottom.  Over time the waterfall plot will fill the screen, with the most recent measurements at the bottom and oldest at the top.  By looking at how the color of the watefall plot changes, you can see how the intensity of a radio signal changes over time.

See the image below for the full range of intensity colors:

To switch back to the instantaneous frequency graph press the SWITCH MODE button at the top of the screen again.  You can press SWITCH MODE to swap between instantaneous and waterfall plot modes.

Another useful feature of the graph is a full screen view.  Press anywhere on the graph image itself, like the middle of the screen.  You should see the graph maximize itself in a full screen view with no buttons or labels.  For example this is a full screen of the instantaneous mode:

The fullscreen view is great if you want to more clearly see the graph.  You can switch back to the normal view by pressing the middle of the graph again.

Now go back to the settings menu by pressing the CONFIG button.  Here's a quick overview of each setting:

  • Center Frequency
    • This is the center frequency of the tuner and must be a value in megahertz.  The exact range of allowed values will depend on the radio tuner, but for the RTL-SDR in the store the range is about 24 Mhz to 1,850 Mhz.  Note that it can be illegal to tune certain frequencies so check the laws in your country!
  • Sample Rate
    • This controls how wide the range of frequencies are that will be displayed in the graph.  The tuner doesn't support a large range of sample rates so your best bet is to stick with the default of 2.4 Mhz.
  • Gain
    • The internal gain of the tuner can be adjusted with this setting.  By increasing the gain you can remove noise and make a weak signal more easily visible.  The tuner has a limited range of gains from about 1 to 50 dB and will try to use a value as close to the configured gain as possible.  You can also choose AUTO as a gain option to have the tuner automatically adjust the gain for the best signal reception.  I recommend sticking with auto gain for simplicity.
  • Min and Max
    • These values at the bottom of the setting list control the minimum and maximum intensity values of the graph.  Like gain they can be set to AUTO, in which case they will adjust themselves based on the lowest or highest intensity values ever seen.  However if you want to restrict the graph to a certain range of values you can set explicit intensity values in decibels.  I've found a range of -3 dB to 50 dB is a good general range to set when using automatic gain.

Finally if you want to exit the tool, in the main graph view press the QUIT button in the upper right.  A confirmation dialog will show which you can accept to exit, or cancel to return back to the tool.

That's all there is to using the Freq Show Raspberry Pi RTL-SDR scanner tool!  If you run into issues with the tool or wish to contribute to it, check out the tool's home on GitHub.

Have fun exploring the world of radio waves around you!

This guide was first published on Oct 02, 2014. It was last updated on 2022-06-23 16:08:32 -0400.