You turn the dial on your radio, and hear a powerful source of interference crackle in over the baseline noise. You’re interested as to where it might be coming from. You’re receiving it well, and the signal strength is strong, but is that because it’s close or just particularly powerful? What could it be? How would you even go about tracking it down?
When it comes to hunting down radio transmissions, Justin McAllister and Nick Foster have a great deal of experience in this regard. They came down to the 2024 Hackaday Superconference to show us how it’s done.
Transmissions From Where?
Nick Foster opens the talk by discussing how the first job is often to figure out what you’re seeing when you pick up a radio transmission. “The moral of this talk is that your hardware is always lying to you,” says Nick. “In this talk, we’re going to show you how your radio lies to you, what you can do about it, and if your hardware is not lying to you, what is that real station that you’re looking at?” It can be difficult to tease out the truth of what the radio might seem to be picking up. “How do we determine what a signal actually is?” he asks. “Is it a real signal that we’re looking at which is being transmitted deliberately from somebody else, or is it interference from a bad power supply, or is it a birdie—a signal that’s created entirely within my own radio that doesn’t exist at all?”
There are common tools used to perform this work of identifying just what the radio is actually picking up and where it’s coming from. Justin goes over some of the typical hardware, noting that the RX-888 is a popular choice for software-defined radio that can be tuned across HF, VHF, and UHF bands. It’s highly flexible, and it’s affordable to boot, as is the Web-888 which can be accessed conveniently over a web browser. Other common SDRs are useful, too, as are a variety of filters that can aid with more precise investigations.
Justin demonstrates an errant radio emission from the brushed motor in his furnace, noting how it varies in bandwidth—a surefire tell versus intentional radio transmissions.
Establishing a grounding in reality is key, Justin steps up to explain. “We turn our SDR on, we stick [on] the little antenna that comes with it, and we start looking at something,” says Justin. “Are the signals that we see there actually real?” He notes that there are some basics to consider right off the bat. “One key point to make is that nobody makes money or has good communication using an unmodulated carrier,” he points out. “If you just see a tone somewhere, it might be real, but there’s a good chance that it’s not.”
It’s perhaps more likely unintentional radiation, noise, or something generated inside the hardware itself on your end. It’s also worth looking at whether you’re looking at a fixed frequency or a changing frequency to pin things down further. Gesturing to a spectrogram, he notes that the long, persistent lines on the spectrogram are usually clues to more intentional transmissions. Intermittent squiggles are more often unintentional. Justin points at some that he puts down to the emissions from arc welders, sparking away as they do, and gives an example of what emissions from typical switching power supplies look like.
There are other hints to look out for, too. Real human-made signals tend to have some logic to them. Justin notes that real signals usually make “efficient” use of spectrum without big gaps or pointless repetition. It’s also possible to make judgement calls as to whether a given signal makes sense for the band it appears to be transmitted in. Schedule can be a tell, too—if a signal always pops up when your neighbor gets home at 6 PM, it might just be coming from their garage door remote. Justin notes a useful technique for hunting down possible nearby emitters—”Flipping on and off switches is a real good way of figuring out—is it close to me or not?”
SDRs are hugely flexible, but they also have very open front-ends that can lead to some confusing output.
Nick follows up by discussing the tendency of sampling radios to show up unique bizarre transmissions that aren’t apparent on an analog receiver. “One of the curses of the RTL-SDR is actually one of its strengths… it has a completely wide open front end,” notes Nick. “Its ADC which is sampling and capturing the RF has basically nothing except an amplifier in between it and whatever crud you’re putting into it.” This provides great sensitivity and frequency agility, but there’s a catch—”It will happily eat up and spit out lots of horrible stuff,” says Nick. He goes on to explain various ways such an SDR might lie to the user. A single signal might start popping up all over the frequency band, or interfere with other signals coming in from the antenna. He also highlights a great sanity check for hunting down birdies—”If it’s always there, if it’s unchanging, if you unplug your antenna and you still hear it—it’s probably generated in your radio!”
The rest of the talk covers locating transmissions—are they in your house, in the local community, or from even farther afield? It explores the technique of multilateration, where synchronized receivers and maths are used to measure the time differences seen in the signal at each point to determine exactly where a transmission is coming from. The talk also goes over common sources of noise in residential settings—cheap PWM LED lights, or knock-off laptop chargers being a prime example in Nick’s experience. There’s also a discussion of how the noise floor has shifted up a long way compared to 50 years ago, now that the world is full of so many more noise-emitting appliances.
Ultimately, the duo of Justin and Nick brought us a great pun-filled talk on sleuthing for the true source of radio transmissions. If you’ve ever wondered about how to track down some mystery transmitter, you would do well to watch and learn from the techniques explored within!
This articles is written by : Nermeen Nabil Khear Abdelmalak
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