Communicating with space-based ham radio satellites might sound like it’s something that takes a lot of money, but in reality it’s one of the more accessible aspects of the hobby. Generally all that’s needed is a five-watt handheld transceiver and a directional antenna. Like most things in the ham radio world, though, it takes a certain amount of skill which can’t be easily purchased. Most hams using satellites like these will rely on some software to help track them, which is where this new program from [Alex Shovkoplyas] comes in.
The open source application is called SkyRoof and provides a number of layers of information about satellites aggregated into a single information feed. A waterfall diagram is central to the display, with not only the satellite communications shown on the plot but information about the satellites themselves. From there the user can choose between a number of other layers of information about the satellites including their current paths, future path prediction, and a few different ways of displaying all of this information. The software also interfaces with radios via CAT control, and can even automatically correct for the Doppler shift that is so often found in satellite radio communications.
For any ham actively engaged in satellite tracking or space-based repeater communications, this tool is certainly worth trying out. Unfortunately, it’s only available for Windows currently. For those not looking to operate under Microsoft’s thumb, projects such as DragonOS do a good job of collecting up the must-have Linux programs for hams and other radio enthusiasts.
Software Defined Radio (SDR) is the big thing these days, and why not? A single computer can get rid of a room full of boat anchors, and give you better signal discrimination than all but the best kit. Any SDR project needs an RF receiver, and in this project [mircemk] used a single 6J1 vaccum tube to produce an SSB SDR that combines the best of old and new.
Single-tube radios are a classic hack, and where a lot of hams got started back in the day, but there is a reason more complicated circuits tend to be used. On the other hand, if you can throw a PC worth of signal processing at the output, it looks like you can get a very sensitive and selective single-sideband (SSB) receiver.
The 6J1 tube is convenient, since it can run on only 6 V (or down to 3.7 as [mircemk] demonstrates). Here it is used as a mixer, with the oscillator signal injected via the screen grid. Aside from that, the simple circuit consists of a receiving coil, a few resistors and a variable capacitor. How well does it work? Quite well, when paired with a PC; you can judge for yourself in the video embedded below.
Usually when we see a project using a software-defined radio (SDR), the SDR’s inputs and outputs are connected to antennae, but [FromConceptToCircuit]’s project connected an ADALM-Pluto SDR to an RF bridge and a few passive components to make a surprisingly effective network analyzer (part two of the video).
The network analyzer measures two properties of the circuit to which it is connected: return loss (S11) and insertion gain or loss (S21). To measure S21, the SDR feeds a series of tones to the device under test, and reads the device’s output from one of the SDR’s inputs. By comparing the amplitude of the input to the device’s output, a Python program can calculate S21 over the range of tested frequencies. To find S11, [FromConceptToCircuit] put an RF bridge in line with the device being tested and connected the bridge’s output to the SDR’s second input. This allowed the program to calculate the device’s impedance, and from that S11.
The RF bridge and other components introduce some inaccuracies to the measurements, so before making any other measurements, the system is calibrated with both a through connection and an open circuit in place of the tested device. The RF bridge’s directivity was the biggest limiting factor; transfer back from the bridge’s output line caused the reflection under load to exceed the reflection of an open circuit in some frequency ranges, at which point the analyzer couldn’t accurately operate.
[FromConceptToCircuit] was eventually able to make measurements throughout most of the 0.1-3 GHz range with a dynamic range of at least 10 dB, and expects a more directive RF bridge to give even better results. If you’d like to repeat the experiment, he’s made his Python program available on GitHub.
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
