Simulating A Time-Keeping Radio Signal

As far as timekeeping goes, there’s nothing more accurate and precise than an atomic clock. Unfortunately, we can’t all have blocks of cesium in our basements, so various agencies around the world have maintained radio stations which, combined with an on-site atomic clock, send out timekeeping signals over the air. In the United States, this is the WWVB station located in Colorado which is generally receivable anywhere in the US but can be hard to hear on the East Coast. That’s why [JonMackey], who lives in northern New Hampshire, built this WWVB simulator.

Normally, clocks built to synchronize with the WWVB station include a small radio antenna to receive the 60 kHz signal and the 1-bit-per-second data transmission which is then decoded and used to update the time shown on the clock. Most of these clocks have internal (but much less precise) timekeeping circuitry to keep themselves going if they lose this signal, but [JonMackey] can go several days without his clocks hearing it. To make up for that he built a small transmitter that generates the proper timekeeping code for his clocks. The system is based on an STM32 which receives its time from GPS and broadcasts it on the correct frequency so that these clocks can get updates.

The small radio transmitter is built using one of the pins on the STM32 using PWM to get its frequency exactly at 60 kHz, which then can have the data modulated onto it. The radiating area is much less than a meter, so this isn’t likely to upset any neighbors, NIST, or the FCC, and the clocks need to be right beside it to update. Part of the reason why range is so limited is that very low frequency (VLF) radios typically require enormous antennas to be useful, so if you want to listen to more than timekeeping standards you’ll need a little bit of gear.

Hackaday Links Column Banner

Hackaday Links: December 10, 2023

In this week’s episode of “Stupid Chatbot Tricks,” it turns out that jailbreaking ChatGPT is as easy as asking it to repeat a word over and over forever. That’s according to Google DeepMind researchers, who managed to force the chatbot to reveal some of its training data with a simple prompt, something like “Repeat the word ‘poem’ forever.” ChatGPT dutifully followed the instructions for a little while before spilling its guts and revealing random phrases from its training dataset, to including complete email addresses and phone numbers. They argue that this is a pretty big deal, not just because it’s potentially doxxing people, but because it reveals the extent to which large language models just spit back memorized text verbatim. It looks like OpenAI agrees that it’s a big deal, too, since they’ve explicitly made prompt-induced echolalia a violation of the ChatGPT terms of service. Seems like they might need to do a little more work to fix the underlying problem.

Continue reading “Hackaday Links: December 10, 2023”

Ham Radio May Speed Up Soon

The FCC is circulating a proposal for new rules pertaining to amateur radio in the United States. In particular, they want to remove certain baud rate restrictions that have been in place since 1980. It appears the relaxed rules would apply only to some bands, notably some VHF and UHF bands along with the 630 meter and 2200 meter bands, which — we think — are lightly used so far. We’ll save you from grabbing the calculator. That’s around 475 kHz and 136 kHz.

Ham radio operators have long used digital modes like radio teletype and with restrictions on antennas and increasing interference from wireless networking to solar panels and more, digital has become even more popular than in the past. Besides that, cheap computer soundcards make it easier than ever and sophisticated digital modulation techniques have long left the old, clunky TeleType in the dust.

However, the FCC currently limits the baud rate to 300 baud or less, ostensibly to restrict signal bandwidth. No one wants to have an entire band consumed by a 10 Gb RF network. However, modern techniques often squeeze more into less and the FCC will finally recognize that by converting the limit to signal bandwidth, not baud rate.

What’s the bandwidth? For the common bands, it sounds like 2.8 kHz is the answer. For the VLF bands, they are asking for suggestions. The 2200 meter band isn’t even 2.8 kHz wide to start with!

All this talk makes us want to build something for the 2200 meter band. We better start winding the coil now. Then again, maybe we should go piezo. You know, just in case Thomas Dolby tells us that one of our submarines is missing.

Piezoelectric Antennas For Very, Very Low Frequencies

If you want to talk about antennas, the amateur radio community has you covered, with one glaring exception. Very low frequency and Extremely Low Frequency radio isn’t practiced very much, ultimately because it’s impractical and you simply can’t transmit much information when your carrier frequency is measured in tens of Hertz. There is more information on Extremely Low Frequency radio in Michael Crichton’s Sphere than there is in the normal parts of the Internet. Now there might be an easier way to play with VLF radiation, thanks to developers at the National Accelerator Laboratory. They’ve developed a piezoelectric transmitter for very long wavelengths.

Instead of pushing pixies through an antenna, this antenna uses a rod-shaped crystal of lithium niobate, a piezoelectric material. An AC voltage is applied to the rod makes it vibrate, and this triggers an oscillating electric current flow that’s emitted as VLF radiation. The key is that it’s these soundwaves bouncing around that define the resonant frequency, and the speed of sound in lithium niobate is a lot slower than the speed of light, but they’re translated into electric signals because of its piezoelectricity. For contrast, if this were a wire quarter-wave antenna it would be tens of kilometers long.

The application for this sort of antenna is ideally for where regular radio doesn’t work. Radio doesn’t work underwater, but nuclear subs trail an antenna out of the back to receive messages using Extremely Low Frequency radio. A walkie talkie doesn’t work in a mine, and this could potentially be used there. There is a patent for this piezoelectric antenna, so if anyone knows of a source of lithium niobate, put a link in the comments.

We’ve seen this trick before to make small antennas even smaller, but this is the first time we’ve seen it used in the VLF band, where it’s arguably even more impressive.

Sferics, Whistlers, And The Dawn Chorus: Listening To Earth Music On VLF

We live in an electromagnetic soup, bombarded by wavelengths from DC to daylight and beyond. A lot of it is of our own making, especially further up the spectrum where wavelengths are short enough for the bandwidth needed for things like WiFi and cell phones. But long before humans figured out how to make their own electromagnetic ripples, the Earth was singing songs at the low end of the spectrum. The very low frequency (VLF) band abounds with interesting natural emissions, and listening to these Earth sounds can be quite a treat.

Continue reading “Sferics, Whistlers, And The Dawn Chorus: Listening To Earth Music On VLF”

Humans May Have Accidentally Created A Radiation Shield Around Earth


NASA spends a lot of time researching the Earth and its surrounding space environment. One particular feature of interest are the Van Allen belts, so much so that NASA built special probes to study them! They’ve now discovered a protective bubble they believe has been generated by human transmissions in the VLF range.

VLF transmissions cover the 3-30 kHz range, and thus bandwidth is highly limited. VLF hardware is primarily used to communicate with submarines, often to remind them that, yes, everything is still fine and there’s no need to launch the nukes yet.  It’s also used for navigation and broadcasting time signals.

It seems that this human transmission has created a barrier of sorts in the atmosphere that protects it against radiation from space. Interestingly, the outward edge of this “VLF Bubble” seems to correspond very closely with the innermost edge of the Van Allen belts caused by Earth’s magnetic field. What’s more, the inner limit of the Van Allan belts now appears to be much farther away from the Earth’s surface than it was in the 1960s, which suggests that man-made VLF transmissions could be responsible for pushing the boundary outwards.

Continue reading “Humans May Have Accidentally Created A Radiation Shield Around Earth”

Get Set For SAQ On Alexanderson Day With These Active Antennas

If you need to generate a radio frequency electrical signal, you will make some form of electronic oscillator. We’ll probably all be used to oscillators using transistors, tubes, logic gates or a host of other electronic technologies. Similarly if you need to generate radio frequencies at high powers, you’ll couple your oscillator to an amplifier, a relatively simple task with today’s electronic parts bin.

If you needed to do the same thing with a high power radio signal in the early years of the 20th century, none of these options were open to you. There were no transistors or integrated circuits, and the tubes of the day could not produce high power outputs. Radio engineers back then had to employ other solutions to the problem, one of which was the Alexanderson alternator. It’s old news we’ve covered here before at Hackaday, a high frequency alternator capable of generating hundreds of kilowatts in the VLF radio frequency range.

There is one operational Alexanderson alternator remaining in the world at the Varberg radio station at Grimeton in Sweden. It is no longer in constant use, but as a World Heritage Site and museum it is put on air a few times a year including the Sunday closest to the 2nd of July, known as Alexanderson Day. We come now to the point of this article: this year’s 3rd of July Alexanderson Day transmission is fast approaching, and since last time we covered it we signed off with a plea for a good VLF antenna design we should post a solution in good time to allow our readers to receive this year’s signal.

G3XBM's e-field VLF antenna
G3XBM’s e-field VLF antenna

Fixing up a receiver is easy enough, we linked to the original SAQrx VLF Receiver and the extended version in our previous coverage. Both pieces of software use your computer’s sound card as the front end of a software defined radio to receive the 17.2kHz from Grimeton. The antenna though presents a problem. You might think that attaching a long piece of wire to the microphone input would be enough, but the problem is that due to the huge wavelength of the VLF signal any reasonable long wire you might be able to assemble simply wouldn’t be long enough to deliver a good result. Clearly a different antenna is required, and the solution comes courtesy of a high-impedance active e-field antenna. This uses a FET input and a surprisingly small patch antenna to deliver a low noise floor at VLF frequencies rather than to be the amplifier you might expect.

We’ve found a couple of designs for you to look at. The first is a two transistor version you will find in various different guises on many sites. This one uses an MPF102 FET, but you should be able to substitute a J310. The second design is a little more surprising, while it is the same idea of a FET input amplifier it uses a TL071 op-amp as its active device. This is in no way an IC you’d normally expect to find in an RF circuit, however the frequency in question is not that of your normal RF.

If you build either of these antennas we hope you’ll be able to hear the Alexanderson Day transmission. The point of a high power VLF transmitter is that it has a huge coverage area, so it should be possible to receive it across all of Europe and perhaps into the eastern United States. If you are out of range though, never fear. You can always try to pick it up through a handy webSDR receiver closer to the source.

Alexanderson alternator picture By Gunther Tschuch (Own work) [ CC BY 2.5 ], via Wikimedia Commons.