So we have our RTL2832U device, a makeshift antenna, and all the software required to do some real work with it. Now what? As a quick demonstration of what RTL-SDR is capable of, let’s tune into a local FM radio station (click the image for full size):
Let’s walk through this image to get an idea of what’s going on. The button on the top left of the toolbar is the “Power” button which starts and stops the receiver. Once you click the button, you should immediately hear static and see the displays come to life.
The large white number at the top is the frequency we are tuned to, here, 94.5 MHz. The two large displays below it show there is clear activity at this frequency, shown by the peak in the top display and red lines in the lower “waterfall” display. Generally speaking, this is what we are looking for when exploring the radio spectrum. Walking through channels and looking for obvious signals via these two displays lets you find audio patterns within the sea of static.
On the upper right side of Gqrx, you can see the “Receiver Options” panel. This lets you adjust which mode the radio is in (such as AM, FM, CW, etc) and make changes to the automatic gain control (AGC). Most of the time you won’t need to change these settings very often, as most of the interesting stuff is in FM anyway. The most common change you will make here is switching between the two FM modes, “FM-W” and “FM-N” (broadcast radio stations need FM-W, most everything else will be FM-N).
Also take note of the squelch (SQL) slider, this allows you to cut out the audio unless it contains a sufficiently strong signal. The farther to the right you slide it, the stronger a signal will need to be to trigger the audio feed. This can be useful for things such as listening to ham radio broadcasts; by preventing loud bursts of static in between transmissions. Use caution with the squelch however, as turning it too high can mute weak signals completely.
The lower right side contains the “Audio” controls. The most important thing here is the “Gain” slider, which is essentially the volume. Turn this up to hear more of the signal, which may be required for weaker transmissions.
Knowing the basic controls of Gqrx, you should be able to tune the radio to different frequencies and begin exploring the local RF spectrum.
The previous image showed a very close and powerful transmitter, so accordingly the displays show a massive signal. For other broadcasts, it will not be nearly as obvious. Some signals may only appear as a line only a few pixels wide in the waterfall. When hunting for interesting signals, keep an eye out for any activity on the lower waterfall display.
To give you an idea of how this all comes together, the following video shows the basic principles of operation for Gqrx; first setting the frequency in the FM range and listening to some local broadcasts, then switching over to 930 MHz to listen in on some pager transmissions:
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From this point on, things are pretty much up to you. You now possess a software defined radio setup that would have cost hundreds of dollars just a year ago. Once you’ve become comfortable with tuning the radio and recognizing signals, the only limit to your explorations is your curiosity and patience.
Try looking at the frequencies your devices transmit on, see if you can find the signals for things like wireless weather stations or the key fob for your car. What kind of interesting things can you find?
If you’re looking for a challenge, get an antenna (or build one) that’s strong enough to receive signals from the International Space Station or even a geostationary satellite. The timing and alignment for communications at that range takes practice and dedication, and can offer a challenge to even accomplished radio operators.
With powerful software like GNU Radio and Gqrx backed up by affordable hardware such as the Realtek RTL2832U, the sky literally is the only limit to what you can do.