A Variable Capacitor For Not A Lot

There’s one component which used to be ubiquitous in every experimenter’s junk box, but nowadays unless you happen to be a radio amateur the chances are you may not have seen one in a long time, if ever. We’re talking of course about the air-dielectric variable capacitor, the tuning element for millions of radio receivers back in the day but now long ago replaced by much flimsier polymer-dielectric parts. There’s still a need for variable capacitors though, in particular a high-voltage variant for use in magnetic loop antennas. It’s something that [Ben] had a need for, which he solved with a clever combination of PCB material and 3D printing.

While the variable capacitors of yore invariably used intersecting vanes on a rotor, this one has two large parallel plates that intersect as one is moved over the other with a lead screw. It’s cheap and effective, and best of all, the files to make it can be downloaded from Thingiverse. He claims a 34pF-164pF capacitance range, which, looking at the size of the plates we find to be believable (and which is a useful range for most HF applications). We like this solution, and believe it makes more sense than being scalped for an older example at a radio rally.

This isn’t the first variable capacitor we’ve shown you, though some previous examples have been more conventional.

The Battlefield That’s 5 KHz Wide

The airwaves are full of news from the battle in Ukraine, with TV and radio journalists providing coverage at all hours. But for those with a bit of patience there’s something else from the conflict that can be found with a radio receiver, the battle over 5 kHz of spectrum starting at 4625 kHz. This has for many years been the location on the dial for “the Buzzer“, a Russian military transmitter whose nickname describes its monotonous on/off buzzing transmission perfectly. As the current Ukrainian situation has taken shape it has become a minor battleground, and the Buzzer now shares its frequency with a variety of other stations broadcasting music, spectrograms, and other radio junk intended to disrupt it.

A spectrogram showing the wavy line of an air raid siren
The air raid siren produces a particularly distinctive spectrogram.

For the curious this can be watched unfolding on a spectrogram or through headphones by anyone within range who has an HF receiver, or for everyone else, with a WebSDR. In Western Europe it’s best listened to in hours of darkness, we suggest you consult the webSDR.org list to see which has the best signal. We’ve heard it on receivers in Poland, Russia, and the ever-reliable uTwente WebSDR in the Netherlands. Over the time we’ve been monitoring it we’ve heard overlaying speech, and music varying from the Soviet and American anthems through dance music and K-pop to 1960s British rock and of course Boney M’s Rasputin, with a few slightly macabre choices such as Final Countdown and an air raid siren. We’ve even heard TV intros from the Benny Hill Show, the A-Team and Mission Impossible, so whoever is doing this has a wide taste.

Alongside the music at about 4628kHz meanwhile we’ve watched a series of spectrogram messages scroll past in Ukrainian, Russian, and English, ranging from “Stop war” to lewd suggestions about the Russian President. It’s fair to say that none of these transmissions have obscured the Buzzer, but they have had the effect of significantly increasing the noise on the channel.

To have a listen yourself, point a receiver within range at the appropriate time of day towards 4625 kHz and select USB demodulation and a 5 kHz bandwidth. Meanwhile, for some background on the Cold War HF relics, have a read about numbers stations.

S15351 tube transmitter

Retro And New Tech Combine In This Hybrid Ham Transmitter

We’ve said it before and we’ll say it again: the best part about holding an amateur radio license is that it lets you build and use your own transmitting equipment. Hams have been doing this for more than a century — indeed, it was once the only way to get on the air — using whatever technology was available. But the mix of technologies in this low-power transmitter for the 80-meter band is something you don’t see every day.

As ham [Helge Fykse (LA6NCA)] describes in the video below, the project began when he came into possession of a bonanza of vacuum tubes — 12A6 tetrodes, specifically. The new-old-stock tubes were perfect for an RF power amplifier, but that left the problem of what to use for an oscillator. [Helge] chose to meld the old with the new and used oscillator board that he designed. The board has an ATmega88 microcontroller and an Si5351 oscillator, along with a 3V3 regulator to let the module run on 12 volts. And for a nice retro touch, [Helge] put the board in a 3D printed case that looks like an old-fashioned quartz crystal.

There are some other nice design touches here too. A low-pass filter cleans up the harmonics of the oscillator’s 3.5-MHz square wave output before feeding it to the amplifier, in a nod to proper spectrum hygiene. The primary for the amp’s air-core output transformer is hand-wound, with 3D printed spacers to keep the winding neat and even. The tuning process shown below is interesting, and the transmitter was used to make a solid contact with another ham about 100 km away. And we really liked the look of [Helge]’s shack, stuffed as it is with gear both old and new.

We’ve personally tried the Si5351 for QRP transmitters before, but this blend of the old and new really makes us want to find some tubes and get to playing.

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Homemade Panadapter Brings Waterfall To Old Radio

Ham radio operators can be pretty selective about their gear. Some are old-school tube purists who would never think of touching a rig containing transistors, and others are perfectly happy with the small Software Defined Radio (SDR) hooked up to their PC. The vast majority, though, of us are somewhere in between — we appreciate the classic look and feel of vintage radios as well as the convenience of modern ones. Better yet, some of us even like to combine the two by adding a few modern bells and whistles to our favorite “boat anchor.”

[Scott Baker] is one such Ham. He’s only had his license for a few months now and has already jumped into some great projects, including adding a panadapter to an old Drake R-4B Receiver. What’s a panadapter, you may ask? As [Scott] explains in his excellent writeup and video, a panadapter is a circuit that grabs a wideband signal from a radio receiver that typically has a narrowband output. The idea is that rather than just listen to somebody’s 4kHz-wide transmission in the 40m band, you can listen to a huge swath of the spectrum, covering potentially hundreds of transmissions, all at the same time.

Well, you can’t actually listen to that many transmissions at once — that would be a garbed mess. What you can do with that ultrawide signal, however, is look at it. If you take an FFT of the signal to put it in the frequency domain (by using a spectrum analyzer, or in [Scott]’s case, an SDR), you can see all sorts of different signals up and down the spectrum. This makes it a heck of a lot easier to find something to listen to — rather than spinning the dial for hours, hoping to come across a transmission, you can just see where all of the interesting signals are.

This isn’t the first (or even the twentieth) time that [Scott]’s work has graced our pages, so make sure to check some of his other incredible projects in our archives!

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Mon Dieu! French Parent Kills Cell Service For An Entire Town To Stop Kids Surfing

It used to be that having technical skills meant that fixing the computer problems of elderly relatives was a regular occurrence. Over the last few years this has been joined by another request on our time; friends with teenage children requesting help configuring their routers such that Internet access is curtailed when the kids should sleeping. In France a desperate parent took more extreme measures, buying a wideband frequency jammer to ensure les petits anges can’t waste the night away on social media sites through their cellular connections. It had the intended effect, but sadly it also interrupted cellular coverage over a wide area The French spectrum regulator ANFR sent in their investigators (French, Google Translate link), and now the unfortunate parent faces the prospect of up to 6 months imprisonment and €30,000 fine for owning and using a device that’s illegal in France.

A cursory search of everybody’s favourite online electronics bazaars will find plenty of these devices, so perhaps what’s surprising is that we don’t see more of these devices even if it’s not the first tale of interference tracking that we’ve seen. Judging by the strategies our friends with kids take, we’d suggest meanwhile to the unfortunate French person, that they simply equip their kids with restricted data plans.

Machining Waveguides For 122 GHz Operation Is Delicate Work

Millimeter-wave Radars used in modern cars for cruise control and collision avoidance are usually designed to work at ranges on the order of 100 meters or so. With some engineering nous, however, experimenters have gotten these devices sending signals over ranges of up to 60 km in some tests. [Machining and Microwaves] decided to see if he could push the boat out even further, and set out machining some waveguide combiner cavities so he could use the radar chips with some very high-performance antennas.

Precision-machined components are required to successfully use these 122 GHz components for long-range transmissions.

The end goal of the project is to produce a 53 dBi antenna for the 122GHz signal put out by the mmWave radar chips commonly found in automotive applications. Working at this frequency requires getting tolerances just so in order to create an antenna that performs well.

Plenty of fine lathe work and cheerful machining banter later, and the precision waveguide is done. It may not look like much to the untrained eye, but much careful design and machining went on to make it both easy to attach to the radar and parabolic antenna system, and to make it perform at a high enough level to hopefully break records set by other enterprising radio experimenters. If that wasn’t all hard enough, though, the final job involved making 24 of these things!

There aren’t a whole lot of microwave antenna-specific machining channels on YouTube, so if you’ve been thirsty for that kind of content, this video is very much for you. If you’re more interested in antennas for lower frequencies, though, you might find some of our other stories to your liking. Video after the break.

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Ask Hackaday: What’s Going On With Mazdas In Seattle?

What hacker doesn’t love a puzzle? We have a doozy for you. According to KUOW — the NPR affiliate in Seattle — they have been getting an unusual complaint. Apparently, if you drive a Mazda made in 2016 and you tune to KUOW, your radio gets stuck on their frequency, 94.9 MHz, and you can’t change it.

According to a post from the radio station, it doesn’t just affect the FM radio. A listener named Smith reported:

“I tried rebooting it because I’ve done that in the past and nothing happened,” Smith said, “I realized I could hear NPR, but I can’t change the station, can’t use the navigation, can’t use the Bluetooth.”

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