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.
The world’s militaries have always been at the forefront of communications technology. From trumpets and drums to signal flags and semaphores, anything that allows a military commander to relay orders to troops in the field quickly or call for reinforcements was quickly seized upon and optimized. So once radio was invented, it’s little wonder how quickly military commanders capitalized on it for field communications.
Radiotelegraph systems began showing up as early as the First World War, but World War II was the first real radio war, with every belligerent taking full advantage of the latest radio technology. Chief among these developments was the ability of signals in the high-frequency (HF) bands to reflect off the ionosphere and propagate around the world, an important capability when prosecuting a global war.
But not long after, in the less kinetic but equally dangerous Cold War period, military planners began to see the need to move more information around than HF radio could support while still being able to do it over the horizon. What they needed was the higher bandwidth of the higher frequencies, but to somehow bend the signals around the curvature of the Earth. What they came up with was a fascinating application of practical physics: meteor burst communications.
In these days of everything-streaming, it’s great to see an old school radio build. It’s even better when it’s not old-school at all, but packed full of modern ICs and driven by a micro-controller like the dsPIC in [Minh Danh]’s dsMP3 build. Best of all is when we get enough details that the author needs two blog posts — one for hardware, and one for firmware — like [Minh Danh] has done.
This build does it all: radio, MP3 playback, and records incoming signals. The radio portion of the build is driven by an Si4735, which allows for receiving both in FM and AM — with all the AM bands, SW, MW and LW available. The FM section does support RDS, though because [Minh Danh] ran out of pins on the dsPIC, isn’t the perfect implementation.
Just look at that thru-hole goodness.
The audio section is a good intro to audio engineering if you’ve never done a project like this: he’s using a TDA1308 for headphones, which feeds into a NS8002 to drive some hefty stereo speakers– and he tells you why he selected those chips, as well as providing broken-out schematics for each. Really, we can’t say enough good things about this project’s documentation.
That’s before we get to the firmware, where he tells us how he manages to get the dsPIC to read out MP3s from a USB drive, and write WAVs to it. One very interesting detail is how he used the dsPIC’s ample analog inputs to handle the front panel buttons on this radio: a resistor ladder. It’s a great solution in a project that’s full of them.
Of course we’ve seen radio receivers before, and plenty of MP3 players, too — but this might be the first time we’ve seen an electronic Swiss army knife with all these features, and we’re very glad [Minh Danh] shared it with us.
One of the first things that an amateur radio operator is likely to do once receiving their license is grab a dual-band handheld and try to make contacts with a local repeater. After the initial contacts, though, many hams move on to more technically challenging aspects of the hobby. One of those being activating space-based repeaters instead of their terrestrial counterparts. [saveitforparts] takes a look at some more esoteric uses of these radio systems in his latest video.
There are plenty of satellite repeaters flying around the world that are actually legal for hams to use, with most being in low-Earth orbit and making quick passes at predictable times. But there are others, generally operated by the world’s militaries, that are in higher geostationary orbits which allows them to serve a specific area continually. With a specialized three-dimensional Yagi-Uda antenna on loan, [saveitforparts] listens in on some of these signals. Some of it is presumably encrypted military activity, but there’s also some pirate radio and state propaganda stations.
There are a few other types of radio repeaters operating out in space as well, and not all of them are in geostationary orbit. Turning the antenna to the north, [saveitforparts] finds a few Russian satellites in an orbit specifically designed to provide polar regions with a similar radio service. These sometimes will overlap with terrestrial radio like TV or air traffic control and happily repeat them at brief intervals.
[Bill Dudley] had a problem. He had an Onkyo AV receiver that did a great job… until it didn’t. A DSP inside failed. When that happened, the main microprocessor running the show decided it wouldn’t play ball without the DSP operational. [Bill] knew the bulk of the audio hardware was still good, it was just the brains that were faulty. Thus started a 4-month operation to resurrect the Onkyo receiver with new intelligence instead.
[Bill’s] concept was simple. Yank the dead DSP, and the useless microprocessor as well. In their place, an ESP32 would be tasked with running things. [Bill] no longer cared if the receiver had DSP abilities or even the ability to pass video—he just wanted to use it as the quality audio receiver that it was.
His project report steps through all the hard work he went through to get things operational again. He had to teach the ESP32 to talk to the front panel display, the keys, and the radio tuner. More challenging was the core audio processor—the obscure Renaisys R2A15218FP. However, by persevering, [Bill] was able to get everything up and running, and even added some new functionality—including Internet radio and Bluetooth streaming.
It’s a heck of a build, and [Bill] ended up with an even more functional audio receiver at the end of it all. Bravo, we say. We love to see older audio gear brought back to life, particularly in creative ways. Meanwhile, if you’ve found your own way to save a piece of vintage audio hardware, don’t hesitate to let us know!
The basic concept is simple enough for a generator—magnets moving past coils to generate electricity. Of course, Lego doesn’t offer high-strength magnetic components or copper coils, so this generator is a hybrid build which includes a lot of [Jamie’s] non-Lego parts. Ultimately though, this is fun because of the weird way it’s built. Lego Technic parts make a very crude turbine, but it does the job. The levitation is a particularly nice touch—the build uses magnets to hover the rotor in mid-air to minimize friction to the point where it can free wheel for minutes once run up to speed. The source of power for this contraption is interesting, too. [Jamie] didn’t just go with an air compressor or a simple homebrew soda bottle tank. Instead, he decided to use a couple of gas duster cans to do the job. The demos are pretty fun, with [Jamie] using lots of LEDs and a radio to demonstrate the output. The one thing we’d like to see more of is proper current/voltage instrumentation—and some measurement of the RPM of this thing!
While few of us will be rushing out to build Lego generators, the video nonetheless has educational value from a mechanical engineering standpoint. Fluids and gases really do make wonderful bearings, as we’ve discussed before. Video after the break.
GPS is an incredible piece of modern technology. Not only does it allow for locating objects precisely anywhere on the planet, but it also enables the turn-by-turn directions we take for granted these days — all without needing anything more than a radio receiver and some software to decode the signals constantly being sent down from space. [Chris] took that last bit bit as somewhat of a challenge and set off to write a software-defined GPS receiver from the ground up.
As GPS started as a military technology, the level of precision needed for things like turn-by-turn navigation wasn’t always available to civilians. The “coarse” positioning is only capable of accuracy within a few hundred meters so this legacy capability is the first thing that [Chris] tackles here. It is pretty fast, though, with the system able to resolve a location in 24 seconds from cold start and then displaying its information in a browser window. Everything in this build is done in Python as well, meaning that it’s a great starting point for investigating how GPS works and for building other projects from there.
The other thing that makes this project accessible is that the only other hardware needed besides a computer that runs Python is an RTL-SDR dongle. These inexpensive TV dongles ushered in a software-defined radio revolution about a decade ago when it was found that they could receive a wide array of radio signals beyond just TV.
