Radio Apocalypse: Meteor Burst Communications

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.

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All-Band Radio Records Signals, Plays MP3s

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.

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A man is shown on the left of the screen, speaking to the camera. On the right of the screen, a Smith chart is displayed. At the top of the screen, the words "TWO METHODS" are displayed.

A Gentle Introduction To Impedance Matching

Impedance matching is one of the perpetual confusions for new electronics students, and for good reason: the idea that increasing the impedance of a circuit can lead to more power transmission is frighteningly unintuitive at first glance. Even once you understand this, designing a circuit with impedance matching is a tricky task, and it’s here that [Ralph Gable]’s introduction to impedance matching is helpful.

The goal of impedance matching is to maximize the amount of power transmitted from a source to a load. In some simple situations, resistance is the only significant component in impedance, and it’s possible to match impedance just by matching resistance. In most situations, though, capacitance and inductance will add a reactive component to the impedance, in which case it becomes necessary to use the complex conjugate for impedance matching.

The video goes over this theory briefly, but it’s real focus is on explaining how to read a Smith chart, an intimidating-looking tool which can be used to calculate impedances. The video covers the basic impedance-only Smith chart, as well as a full-color Smith chart which indicates both impedance and admittance.

This video is the introduction to a planned series on impedance matching, and beyond reading Smith charts, it doesn’t really get into many specifics. However, based on the clear explanations so far, it could be worth waiting for the rest of the series.

If you’re interested in more practical details, we’ve also covered another example before. Continue reading “A Gentle Introduction To Impedance Matching”

Radio Repeaters In The Sky

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.

[saveitforparts] has plenty of videos looking at other satellite communications, including grabbing images from Russian weather satellites, using leftover junk to grab weather data from geostationary orbit, and accessing the Internet via satellite with 80s-era technology.

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Open Source DMR Radio

While ham radio operators have been embracing digital mobile radio (DMR), the equipment is most often bought since — at least in early incarnations — it needs a proprietary CODEC to convert speech to digital and vice versa. But [QRadioLink] decided to tackle a homebrew and open source DMR modem.

The setup uses a LimeSDR, GNU Radio, and Codec2. There are some other open DMR projects, such as OpenRTX. So we are hopeful there are going to be more choices. The DMR modem, however, is only a proof-of-concept and reuses the MMDVMHost code to do the data link layer.

[QRadioLink] found several receiver implementations available, but only one other DMR transmitter — actually, a transceiver. Rather than use an AMBE hardware device or the potentially encumbered mbelib codec, the project uses Codec2 which is entirely open source.

There’s a lot of explanation about the data collection to prepare for the project, and then a deep dive into the nuts and bolts of the implementation. You might enjoy the video below to see things in action.

If you just want to listen to DMR, it’s easy. If Codec2 sounds familiar, it is part of M17.

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Brush Up On Your Trade Craft With This Tiny FM Bug

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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Farewell Economy 7, A Casualty Of The Long Wave Switch-Off

If you paid attention to advertising in 1980s Britain, you were never far from Economy 7. It was the magic way to heat your house for less, using storage heaters which would run at night using cheap electricity, and deliver warmth day-long. Behind it all was an unseen force, a nationwide radio switching signal transmitted using the BBC’s 198 kHz Long Wave service. Now in 2025 the BBC Radio 4 Long Wave service it relies on is to be turned off, rendering thousands of off-peak electricity meters still installed, useless. [Ringway Manchester] is here to tell the tale.

The system was rolled out in the early 1980s, and comprised of a receiver box which sat alongside your regular electricity meter and switched in or out your off-peak circuit. The control signal was phase-modulated onto the carrier, and could convey a series of different energy use programs. 198 kHz had the useful property due to its low frequency of universal coverage, making it the ideal choice. As we’ve reported in the past the main transmitter at Droitwich is to be retired due to unavailability of the high-power vacuum tubes it relies on, so now time’s up for Economy 7 too. The electricity companies are slow on the uptake despite years of warning, so there’s an unseemly rush to replace those old meters with new smart meters. The video is below the break.

The earliest of broadcast bands may be on the way out, but it’s not entirely over. There might even be a new station on the dial for some people.

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