spy radio

9 Articles

High-Stakes Fox Hunting: The FCC’s Radio Intelligence Division In World War II

June 4, 2025 by 32 Comments

With few exceptions, amateur radio is a notably sedentary pursuit. Yes, some hams will set up in a national or state park for a “Parks on the Air” activation, and particularly energetic operators may climb a mountain for “Summits on the Air,” but most hams spend a lot of time firmly planted in a comfortable chair, spinning the dials in search of distant signals or familiar callsigns to add to their logbook.

There’s another exception to the band-surfing tendencies of hams: fox hunting. Generally undertaken at a field day event, fox hunts pit hams against each other in a search for a small hidden transmitter, using directional antennas and portable receivers to zero in on often faint signals. It’s all in good fun, but fox hunts serve a more serious purpose: they train hams in the finer points of radio direction finding, a skill that can be used to track down everything from manmade noise sources to unlicensed operators. Or, as was done in the 1940s, to ferret out foreign agents using shortwave radio to transmit intelligence overseas.

That was the primary mission of the Radio Intelligence Division, a rapidly assembled organization tasked with protecting the United States by monitoring the airwaves and searching for spies. The RID proved to be remarkably effective during the war years, in part because it drew heavily from the amateur radio community to populate its many field stations, but also because it brought an engineering mindset to the problem of finding needles in a radio haystack.

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Brush Up On Your Trade Craft With This Tiny FM Bug

April 11, 2025 by 19 Comments

Would-be spooks and spies, take note: this one-transistor FM transmitter is a circuit you might want to keep in mind for your bugging needs. True, field agents aren’t likely to need to build their own equipment, but how cool a spy would you be if you could?

Luckily, you won’t need too many parts to recreate [Ciprian (YO6DXE)]’s project, most of which could be found in a decently stocked junk bin, or even harvested from e-waste. On the downside, the circuit is pretty fussy, with even minor component value changes causing a major change in center frequency. [Ciprian] had to do a lot of fiddling to get the frequency in the FM band, particularly with the inductor in the LC tank circuit. Even dropping battery voltage shifted the frequency significantly, which required a zener diode to address.

[Ciprian] ran a few tests and managed to get solid copy out to 80 meters range, which is pretty impressive for such a limited circuit. The harmonics, which extend up into the ham bands and possibly beyond, are a bit of a problem; while those could be addressed with a low-pass filter, in practical terms, the power of this little fellow is probably low enough to keep you from getting into serious trouble. Still, it’s best not to push your luck.

While you’re trying your hand at one-transistor circuits, you might want to try [Ciprian]’s one-transistor CW transceiver next.

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Spy Radio Setup Gets A Tiny Power Supply For Field Operations

June 29, 2023 by 14 Comments

[Helge Fyske (LA6NCA)] may not be an actual spy — then again, he may be; if he’s good at it, we wouldn’t know — but he has built a couple of neat vacuum tube spy radios in the past. And there’s no better test for such equipment than to haul it out into the field and try to make some contacts. But how do you power such things away from the bench?

To answer that question, skip ahead to the 3:18 mark of the video below, where [Helge] shows off his whole retro rig, including the compact 250-volt power supply he built for his two-tube 80-m Altoids tin spy transceiver. In the shack, [Helge] powers it with a bench power supply of his own design to provide the high anode voltage needed for the tubes, as well as 12 volts for their heaters. Portable operations require a more compact solution, preferably one that can be run off a battery small enough to pack in.

By building his power supply in a tin, [Helge] keeps to his compact build philosophy. But the circuit is all solid state, which is an interesting departure for him. The switch-mode supply uses a 4047 astable multivibrator chip as a 50-kHz oscillator, which switches back and forth between a pair of MOSFETs to drive a transformer. This steps up the 12-volt input to 280 volts AC, which is then rectified, filtered, and regulated to 250 volts DC.

To round out his spy rig, [Helge] also designed a tiny Morse key, which appears to be 3D printed and fits in its own tin, and a compact dipole antenna. Despite picking what appears to be a challenging location — the bottom of a steep-sided fjord — [Helge] was easily able to make contacts over a distance of 400 km. His noise floor was remarkably low, a testament to the solid design of his power supply. Including the sealed lead acid battery, the whole kit is compact and efficient, and it’s a nice example of what vacuum tubes and solid state can accomplish together.

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Spy Transceiver Makes Two Tubes Do The Work Of Five

May 19, 2023 by 26 Comments

Here at Hackaday, we love following along with projects as they progress. That’s especially true when a project makes a considerable leap in terms of functionality from one version to another, or when the original design gets more elegant. And when you get both improved function and decreased complexity at the same time? That’s the good stuff.

Take the recent improvements to a vacuum tube “spy radio” as an example. Previously, [Helge (LA6NCA)] built both a two-tube transmitter and a three-tube receiver, either of which would fit in the palm of your hand. A little higher math seems to indicate that combining these two circuits into a transceiver would require five tubes, but that’s not how hams like [Helge] roll. His 80-m CW-only transceiver design uses only two tubes and a lot of tricks, which we admit we’re still wrapping our heads around. On the receive side, one tube serves as a mixer/oscillator, combining the received signal with a slightly offset crystal-controlled signal to provide the needed beat frequency. The second tube serves as the amplifier, both for the RF signal when transmitting, and for audio when receiving.

The really clever part of this build is that [Helge] somehow stuffed four separate relays into the tiny Altoids tin chassis. Three of them are used to switch between receive and transmit, while the fourth is set up as a simple electromagnetic buzzer. This provides the sidetone needed to effectively transmit Morse code, and is about the simplest way we’ve ever seen to address that need. Also impressive is how [Helge] went from a relatively expansive breadboard prototype to a much more compact final design, and how the solder was barely cooled before he managed to make a contact over 200 km. The video below has all the details.

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Tiny Three-Tube Receiver Completes Spy Radio Suite

April 22, 2023 by 6 Comments

In our surface-mount age, it’s easy to be jaded about miniaturization. We pretty much expect every circuit to be dimensionally optimized, something that’s easy to do when SMDs that rival grains of sand are available. But dial the calendar back half a century or so and miniaturization was a much more challenging proposition.

Challenging, perhaps, but by no means unachievable, as [Helge Fyske (LA6NCA)] demonstrates with this ultra-compact regenerative vacuum tube receiver. It’s a companion to his recent “spy transmitter,” a two-tube radio built in — or on, really — an Altoids tin. The transmitter was actually a pretty simple circuit, just a crystal-controlled oscillator and an RF amplifier really, but still managed about 1.5 Watts output on the 80-meter ham band.

The receiver circuit ended up being much more complicated, as receivers do, and therefore harder to cram into the allotted space. [Helge]’s used a three-tube regenerative design, with one tube each devoted to the RF amp, detector/mixer, and audio amplifier stages. As in the transmitter, the receiver tubes are mounted on the outside of the box, with the inside crammed full of components. [Helge] had to be quite careful about component positioning, to prevent interstage coupling and other undesirable side effects of building in such close quarters.

Was it worth it? Judging by the video below, absolutely! We’ve rarely heard performance like that from even a modern receiver with all the bells and whistles, let alone from a homebrew design under such constraints. It sounds fantastic, and hats off to [Helge] for completing his spy radio suite in style.

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Two-Tube Spy Transmitter Fits In The Palm Of Your Hand

March 29, 2023 by 13 Comments

It’s been a long time since vacuum tubes were cutting-edge technology, but that doesn’t mean they don’t show up around here once in a while. And when they do, we like to feature them, because there’s still something charming, nay, romantic about a circuit built around hot glass and metal. To wit, we present this compact two-tube “spy radio” transmitter.

From the look around his shack — which we love, by the way — [Helge Fykse (LA6NCA)] really has a thing for old technology. The typewriter, the rotary phones, the boat-anchor receiver — they all contribute to the retro feel of the space, as well as the circuit he’s working on. The transmitter’s design is about as simple as can be: one tube serves as a crystal-controlled oscillator, while the other tube acts as a power amplifier to boost the output. The tiny transmitter is built into a small metal box, which is stuffed with the resistors, capacitors, and homebrew inductors needed to complete the circuit. Almost every component used has a vintage look; we especially love those color-coded mica caps. Aside from PCB backplane, the only real nod to modernity in the build is the use of 3D printed forms for the coils.

But does it work? Of course it does! The video below shows [Helge] making a contact on the 80-meter band over a distance of 200 or so kilometers with just over a watt of power. The whole project is an excellent demonstration of just how simple radio communications can be, as well as how continuous wave (CW) modulation really optimizes QRP setups like this.

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Hackaday Links: October 31, 2021

October 31, 2021 by 17 Comments

Global supply chain issues are beginning to hit closer to home for the hacker community, as Raspberry Pi has announced their first-ever price increase on their flagship Pi 4. The move essentially undoes the price drop on the 2GB version of the Pi 4 that was announced in February, and sets the price back up from $35 to $45. Also rolled back is the discontinuation of the 1GB version, which will now be available at the $35 price point. The announcements come from Eben Upton himself, who insists the price increase is only temporary. We applaud his optimism, but take it with a grain of salt since he also said that 2021 production across the board will stay at the seven million-unit level, which is what they produced in 2020. That seems to speak to deeper issues within the supply chain, but more immediately, it’s likely that the supply of Pi products will be pinched enough that you’ll end up paying above sticker price just to get the boards you need. Hope everyone is stocked up.

On the topic of supply chain issues and their threat to Christmas gift-giving, here’s one product we hope is stranded in a container off Long Beach or better still, bobbing along in the Strait of Juan De Fuca: a toddler’s toy telephone that actually makes and receives calls. Anyone born in the last 60 years probably had one of the Fisher-Price Chatter telephone, a toy that in its original form looked like a desk telephone on wheels that was dragged behind the child, popping along and providing endless hours of clicky amusement as kids twisted the dial and lifted the receiver. Come to think of it, the Chatter telephone may be as close to a dial phone as anyone born since 1990 may have come. Anyway, some genius stuck a Bluetooth module into the classic phone to let it hook up to an app on an actual phone, allowing kids (or more likely their nostalgia-soaked parents) to make and receive calls. It’s actually priced at a reasonable $60, so there might be some hacking potential here.

Also tangential to supply chains, we stumbled across a video guide to buying steel that might interest readers. Anyone who has seen the displays of steel and other metals at the usual big-box retailers might wonder what the fuss is, but buying steel that way or ordering online is a great way to bust a project’s budget. Fabricator and artist Doug Boyd insists that finding a local steel supplier is the best bang for your buck, and has a bunch of helpful tips for not sounding like a casual when you’re ordering. It’s all good advice, and would have helped us from looking foolish a time or two at the metal yard; just knowing that pipe is measured by inside diameter while tubing is measured by outside dimensions is worth the price of admission alone.

With all the money you save on steel and by not buying Raspberry Pis, perhaps you’ll have a couple of hundred thousand Euros lying around to bid on this authentic 1957 Sputnik I satellite. The full-scale model of Earth’s first artificial satellite — manhole covers excluded — was a non-flown test article, but externally faithful to the flown hardware that kicked off the first Space Race. The prospectus says that it has a transmitter and a “modern power supply”; it’s not clear if the transmitter was originally part of the test article or added later. The opening bid is €85,000 and is expected to climb considerably.

And finally, there’s something fascinating about “spy radios,” especially those from the Cold War era and before, when being caught with one in your possession was probably going to turn out to be a very bad day. One such radio is the Radio Orange “Acorn” receiver, which is in the collection of the Crypto Museum. The radio was used by the Dutch government to transmit news and information into the occupied Netherlands from their exile in London. Built to pass for a jewelry box, the case for the radio was made from an old cigar box and is a marvel of 1940s miniaturization. The radio used three acorn-style vacuum tubes and was powered by mains current; another version of the Radio Orange receiver was powered by a bike dynamo or even a water-powered turbine, which could be run from a tap or garden hose. The video below shows the water-powered version in action, but the racket it made must have been problematic for its users, especially given the stakes.

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