vlf

13 Articles

Homebrew Sferics Receiver Lets You Tune Into Earth Music

October 30, 2024 by 10 Comments

It probably comes as little surprise that our planet is practically buzzing with radio waves. Most of it is of our own making, with cell phones, microwaves, WiFi, and broadcasts up and down the spectrum whizzing around all the time. But our transmissions aren’t the only RF show in town, as the Earth itself is more than capable of generating radio signals of its own, signals which you can explore with a simple sferics receiver like this one.

If you’ve never heard of sferics and other natural radio phenomena, we have a primer to get you started. Briefly, sferics, short for “atmospherics,” are RF signals in the VLF range generated by the millions of lightning discharges that strike the Earth daily. Tuning into them is a pretty simple proposition, as [DX Explorer]’s receiver demonstrates. His circuit, which is based on a design by [K8TND], is just a single JFET surrounded by a few caps and resistors, plus a simple trap to filter out the strong AM broadcast signals in his area. The output of the RF amplifier goes directly into an audio amp, which could be anything you have handy — but you risk breaking [Elliot]’s heart if you don’t use his beloved LM386.

This is definitely a “nothing fancy” build, with the RF section built ugly style on a scrap of PCB and a simple telescopic whip used for an antenna. Tuning into the Earth’s radio signals does take some care, though. Getting far away from power lines is important, to limit AC interference. [DX Explorer] also found how he held the receiver was important; unless he was touching the ground plane of the receiver, the receiver started self-oscillating. But the pips, crackles, and pings came in loud and clear on his rig; check out the video below for the VLF action.

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Simulating A Time-Keeping Radio Signal

January 14, 2024 by 14 Comments

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.

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Hackaday Links: December 10, 2023

December 10, 2023 by 8 Comments

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.

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Ham Radio May Speed Up Soon

October 29, 2023 by 14 Comments

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

April 16, 2019 by 40 Comments

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

November 6, 2017 by 19 Comments

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.

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Humans May Have Accidentally Created A Radiation Shield Around Earth

May 21, 2017 by 60 Comments

 

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.

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