Any radio amateur will tell you about the spectre of TVI, of their transmissions being inadvertently demodulated by the smallest of non-linearity in the neighbouring antenna systems, and spewing forth from the speakers of all and sundry. It’s very much a thing that the most unlikely of circuits can function as radio receivers, but… teeth? [Ringway Manchester] investigates tales of musical dental work.
Going through a series of news reports over the decades, including one of Lucille Ball uncovering a hidden Japanese spy transmitter, it’s something all experts who have looked at the issue have concluded there is little evidence for. It was also investigated by Mythbusters. But it’s an alluring tale, so is it entirely fabricated? What we can say is that teeth are sensitive to sound, not in themselves, but because the jaw provides a good path bringing vibrations to the region of the ear. And it’s certainly possible that the active chemical environment surrounding a metal filling in a patient’s mouth could give rise to electrical non-linearities. But could a human body in an ordinary RF environment act as a good enough antenna to provide enough energy for something to happen? We have our doubts.
It’s a perennial story (even in fiction), though, and we’re guessing that proof will come over the coming decades. If the tales of dental music and DJs continue after AM (or Long Wave in Europe) transmissions have been turned off, then it’s likely they’re more in the mind than in the mouth. If not, then we might have missed a radio phenomenon. The video is below the break.
The first transistors were point contact devices, not far from the cats-whiskers of early radio receivers. They were fragile and expensive, and their performance was not very high. The transistor which brought the devices to a mass audience through the 1950s and 1960s was the one which followed, the alloy diffusion type. [Play With Junk] has a failed OC71 PNP alloy diffusion transistor, first introduced in 1957, and has cracked it open for a closer look.
Inside the glass tube is a small wafer of germanium crystal, surrounded by silicone grease. It forms the N-type base of the device, with the collector and emitter being small indium beads fused into the germanium. The junctions were formed by the resulting region of germanium/indium alloy. The outside of the tube is pained black because the device is light-sensitive, indeed a version of this transistor without the paint was sold as the OCP71 phototransistor.
Lithium-ion cells deliver very high energy densities compared to many other battery technologies, but they bring with them a danger of fire or explosion if they are misused. We’re mostly aware of the battery conditioning requirements to ensure cells stay in a safe condition, but how much do we know about the construction of the cells as a factor? [Lumafield] is an industrial imaging company, and to demonstrate their expertise, they’ve subjected a large number of 18650 cells from different brands to a CT scan.
The construction of an 18650 sees the various layers of the cell rolled up in a spiral inside the metal tube that makes up the cell body. The construction of this “jellyroll” is key to the quality of the cell. [Lumafield’s] conclusions go into detail over the various inconsistencies in this spiral, which can result in cell failure. It’s important that the edges of the spiral be straight and that there is no electrode overhang. Perhaps unsurprisingly, they find that cheap no-name cells are poorly constructed and more likely to fail, but it’s also interesting to note that these low-quality cells also have fewer layers in their spiral.
We hope that none of you see more of the inside of a cell in real life than you have to, as they’re best left alone, but this report certainly sheds some light as to what’s going on inside a cell. Of course, even the best cells can still be dangerous without protection.
As you might expect, the University of Puerto Rico at Arecibo has a fascination with radio signals from space. While doing research into the legendary “Wow! Signal” detected back in 1977, they realized that the burst was so strong that a small DIY radio telescope would be able to pick it up using modern software-defined radio (SDR) technology.
This realization gave birth to the Wow@Home project, an effort to document both the hardware and software necessary to pick up a Wow! class signal from your own backyard. The University reasons that if they can get a bunch of volunteers to build and operate these radio telescopes, the resulting data could help identify the source of the Wow! Signal — which they believe could be the result of some rare astrophysical event and not the product of Little Green Men.
Ultimately, this isn’t much different from many of the SDR-based homebrew radio telescopes we’ve covered over the years — get a dish, hook your RTL-SDR up to it, add in the appropriate filters and amplifiers, and point it to the sky. Technically, you’re now a radio astronomer. Congratulations. In this case, you don’t even have to figure out how to motorize your dish, as they recommend just pointing the antenna at a fixed position and let the rotation of the Earth to the work — a similar trick to how the legendary Arecibo Observatory itself worked.
The tricky part is collecting and analyzing what’s coming out of the receiver, and that’s where the team at Arecibo hope to make the most headway with their Wow@Home software. It also sounds like that’s where the work still needs to be done. The goal is to have a finished product in Python that can be deployed on the Raspberry Pi, which as an added bonus will “generate a live preview of the data in the style of the original Ohio State SETI project printouts.” Sounds cool to us.
If you’re interested in lending a hand, the team says they’re open to contributions from the community — specifically from those with experience RFI shielding, software GUIs, and general software development. We love seeing citizen science, so hopefully this project finds the assistance and the community it needs to flourish.
For the most part, the Radio Apocalypse series has focused on the radio systems developed during the early days of the atomic age to ensure that Armageddon would be as orderly an affair as possible. From systems that provided backup methods to ensure that launch orders would reach the bombers and missiles, to providing hardened communications systems to allow survivors to coordinate relief and start rebuilding civilization from the ashes, a lot of effort went into getting messages sent.
Strangely, though, the architects of the end of the world put just as much thought into making sure messages didn’t get sent. The electronic village of mid-century America was abuzz with signals, any of which could be abused by enemy forces. CONELRAD, which aimed to prevent enemy bombers from using civilian broadcast signals as navigation aids, is a perfect example of this. But the growth of civil aviation through the period presented a unique challenge, particularly with the radio navigation system built specifically to make air travel as safe and reliable as possible.
Balancing the needs of civil aviation against the possibility that the very infrastructure making it possible could be used as a weapon against the U.S. homeland is the purpose of a plan called Security Control of Air Traffic and Air Navigation Aids, or SCATANA. It’s a plan that cuts across jurisdictions, bringing military, aviation, and communications authorities into the loop for decisions regarding when and how to shut down the entire air traffic system, to sort friend from foe, to give the military room to work, and, perhaps most importantly, to keep enemy aircraft as blind as possible.
Highways in the Sky
As its name suggests, SCATANA has two primary objectives: to restrict the availability of radio navigation aids during emergencies and to clear the airspace over the United States of unauthorized traffic. For safety’s sake, the latter naturally follows the former. By the time the SCATANA rules were promulgated, commercial aviation had become almost entirely dependent on a complex array of beacons and other radio navigation aids. While shutting those aids down to deny their use to enemy bombers was obviously the priority, safety demanded that all the planes currently using those aids had to be grounded as quickly as possible.
The Rogue Valley VOR station in Table Rock, Oregon. According to the sectional charts, this is a VORTAC station. Source: ZabMilenko, CC BY 3.0, via Wikimedia Commons.
Understanding the logic behind SCATANA requires at least a basic insight into these radio navigation aids. The Federal Communications Commission (FCC) has jurisdiction over these aids, listing “VOR/DME, ILS, MLS, LF and HF non-directional beacons” as subject to shutdown in times of emergency. That’s quite a list, and while the technical details of the others are interesting, particularly the Adcock LF beacon system used by pilots to maneuver onto a course until alternating “A” and “N” Morse characters merged into a single tone, but for practical purposes, the one with the most impact on wartime security is the VOR system.
VOR, which stands for “VHF omnidirectional range,” is a global system of short-range beacons used by aircraft to determine their direction of travel. The system dates back to the late 1940s and was extensively built out during the post-war boom in commercial aviation. VOR stations define the “highways in the air” that criss-cross the country; if you’ve ever wondered why the contrails of jet airliners all follow similar paths and why the planes make turns at more or less the same seemingly random point in the sky, it’s because they’re using VOR beacons as waypoints.
In its simplest form, a VOR station consists of an omnidirectional antenna transmitting at an assigned frequency between 108 MHz and 117.95 MHz, hence the “VHF” designation. The frequency of each VOR station is noted on the sectional charts pilots use for navigation, along with the three-letter station identifier, which is transmitted by the station in Morse so pilots can verify which station their cockpit VOR equipment is tuned to.
Each VOR station encodes azimuth information by the phase difference between two synchronized 30 Hz signals modulated onto the carrier, a reference signal and a variable signal. In conventional VOR, the amplitude-modulated variable signal is generated by a rotating directional antenna transmitting a signal in-phase with the reference signal. By aligning the reference signal with magnetic north, the phase angle between the FM reference and AM variable signals corresponds to the compass angle of the aircraft relative to the VOR station.
More modern Doppler VORs, or DVORs, use a ring of antennas to electronically create the reference and variable signals, rather than mechanically rotating the antenna. VOR stations are often colocated with other radio navigation aids, such as distance measuring equipment (DME), which measures the propagation delay between the ground station and the aircraft to determine the distance between them, or TACAN, a tactical air navigation system first developed by the military to provide bearing and distance information. When a VOR and TACAN stations are colocated, the station is referred to as a VORTAC.
Shutting It All Down
At its peak, the VOR network around the United States numbered almost 1,000 stations. That number is on the decrease now, thanks to the FAA’s Minimum Operational Network plan, which seeks to retire all but 580 VOR stations in favor of cockpit GPS receivers. But any number of stations sweeping out fully analog, unencrypted signals on well-known frequencies would be a bonanza of navigational information to enemy airplanes, which is why the SCATANA plan provides specific procedures to be followed to shut the whole thing down.
Inside the FAA’s Washington DC ARTCC, which played a major role in implementing SCATANA on 9/11. Source: Federal Aviation Administration, public domain.
SCATANA is designed to address two types of emergencies. The first is a Defense Emergency, which is an outright attack on the United States homeland, overseas forces, or allied forces. The second is an Air Defense Emergency, which is an aircraft or missile attack on the continental U.S., Canada, Alaska, or U.S. military installations in Greenland — sorry, Hawaii. In either case, the attack can be in progress, imminent, or even just probable, as determined by high-ranking military commanders.
In both of those situations, military commanders will pass the SCATANA order to the FAA’s network of 22 Air Route Traffic Control Centers (ARTCC), the facilities that handle traffic on the routes defined by VOR stations. The SCATANA order can apply to all of the ARTCCs or to just a subset, depending on the scale of the emergency. Each of the concerned centers will then initiate physical control of their airspace, ordering all aircraft to land at the nearest available appropriate airport. Simultaneously, if ordered by military authority, the navigational aids within each ARTCC’s region will be shut down. Sufficient time is obviously needed to get planes safely to the ground; SCATANA plans allow for this, of course, but the goal is to shut down navaids as quickly as possible, to deny enemy aircraft or missiles any benefit from them.
As for the specific instructions for shutting down navigational aids, the SCATANA plan is understandable mute on this subject. It would not be advisable to have such instructions readily available, but there are a few crumbs of information available in the form of manuals and publicly accessible documents. Like most pieces of critical infrastructure these days, navaid ground stations tend to be equipped with remote control and monitoring equipment. This allows maintenance technicians quick and easy access without the need to travel. Techs can perform simple tasks, such as switching over from a defective primary transmitter to a backup, to maintain continuity of service while arrangements are made for a site visit. Given these facts, along with the obvious time-critical nature of an enemy attack, SCATANA-madated navaid shutdowns are probably as simple as a tech logging into the ground station remotely and issuing a few console commands.
A Day to Remember
For as long as SCATANA has been in effect — the earliest reference I could find to the plan under that name dates to 1968, but the essential elements of the plan seem to date back at least another 20 years — it has only been used in anger once, and even then only partially. That was on that fateful Tuesday, September 11, 2001, when a perfect crystal-blue sky was transformed into a battlefield over America.
By 9:25 AM Eastern, the Twin Towers had both been attacked, American Airlines Flight 77 had already been hijacked and was on its way to the Pentagon, and the battle for United Flight 93 was unfolding above Ohio. Aware of the scope of the disaster, staff at the FAA command center in Herndon, Virginia, asked FAA headquarters if they wanted to issue a “nationwide ground stop” order. While FAA brass discussed the matter, Ben Sliney, who had just started his first day on the job as operations manager at the FAA command center, made the fateful decision to implement the ground stop part of the SCATANA plan, without ordering the shutdown of navaids.
The “ground stop” orders went out to the 22 ARTCCs, which began the process of getting about 4,200 in-flight aircraft onto the ground as quickly and safely as possible. The ground stop was achieved within about two hours without any further incidents. The skies above the country would remain empty of civilian planes for the next two days, creating an eerie silence that emphasized just how much aviation contributes to the background noise of modern life.
[Ralph] is excited about impedance matching, and why not? It is important to match the source and load impedance to get the most power out of a circuit. He’s got a whole series of videos about it. The latest? Matching using a PI network and the venerable Smith Chart.
We like that he makes each video self-contained. It does mean if you watch them all, you get some review, but that’s not a bad thing, really. He also does a great job of outlining simple concepts, such as what a complex conjugate is, that you might have forgotten.
Smith charts almost seem magical, but they are really sort of an analog computer. The color of the line and even the direction of an arrow make a difference, and [Ralph] explains it all very simply.
The example circuit is simple with a 50 MHz signal and a mismatched source and load. Using the steps and watching the examples will make it straightforward, even if you’ve never used a Smith Chart before.
The red lines plot impedance, and the blue lines show conductance and succeptance. Once everything is plotted, you have to find a path between two points on the chart. That Smith was a clever guy.
We looked at part 1 of this series earlier this year, so there are five more to watch since then. If your test gear leaves off the sign of your imaginary component, the Smith Chart can work around that for you.
Communicating with space-based ham radio satellites might sound like it’s something that takes a lot of money, but in reality it’s one of the more accessible aspects of the hobby. Generally all that’s needed is a five-watt handheld transceiver and a directional antenna. Like most things in the ham radio world, though, it takes a certain amount of skill which can’t be easily purchased. Most hams using satellites like these will rely on some software to help track them, which is where this new program from [Alex Shovkoplyas] comes in.
The open source application is called SkyRoof and provides a number of layers of information about satellites aggregated into a single information feed. A waterfall diagram is central to the display, with not only the satellite communications shown on the plot but information about the satellites themselves. From there the user can choose between a number of other layers of information about the satellites including their current paths, future path prediction, and a few different ways of displaying all of this information. The software also interfaces with radios via CAT control, and can even automatically correct for the Doppler shift that is so often found in satellite radio communications.
For any ham actively engaged in satellite tracking or space-based repeater communications, this tool is certainly worth trying out. Unfortunately, it’s only available for Windows currently. For those not looking to operate under Microsoft’s thumb, projects such as DragonOS do a good job of collecting up the must-have Linux programs for hams and other radio enthusiasts.
Software Defined Radio (SDR) is the big thing these days, and why not? A single computer can get rid of a room full of boat anchors, and give you better signal discrimination than all but the best kit. Any SDR project needs an RF receiver, and in this project [mircemk] used a single 6J1 vaccum tube to produce an SSB SDR that combines the best of old and new.
Single-tube radios are a classic hack, and where a lot of hams got started back in the day, but there is a reason more complicated circuits tend to be used. On the other hand, if you can throw a PC worth of signal processing at the output, it looks like you can get a very sensitive and selective single-sideband (SSB) receiver.
The 6J1 tube is convenient, since it can run on only 6 V (or down to 3.7 as [mircemk] demonstrates). Here it is used as a mixer, with the oscillator signal injected via the screen grid. Aside from that, the simple circuit consists of a receiving coil, a few resistors and a variable capacitor. How well does it work? Quite well, when paired with a PC; you can judge for yourself in the video embedded below.
Usually when we see a project using a software-defined radio (SDR), the SDR’s inputs and outputs are connected to antennae, but [FromConceptToCircuit]’s project connected an ADALM-Pluto SDR to an RF bridge and a few passive components to make a surprisingly effective network analyzer (part two of the video).
The network analyzer measures two properties of the circuit to which it is connected: return loss (S11) and insertion gain or loss (S21). To measure S21, the SDR feeds a series of tones to the device under test, and reads the device’s output from one of the SDR’s inputs. By comparing the amplitude of the input to the device’s output, a Python program can calculate S21 over the range of tested frequencies. To find S11, [FromConceptToCircuit] put an RF bridge in line with the device being tested and connected the bridge’s output to the SDR’s second input. This allowed the program to calculate the device’s impedance, and from that S11.
The RF bridge and other components introduce some inaccuracies to the measurements, so before making any other measurements, the system is calibrated with both a through connection and an open circuit in place of the tested device. The RF bridge’s directivity was the biggest limiting factor; transfer back from the bridge’s output line caused the reflection under load to exceed the reflection of an open circuit in some frequency ranges, at which point the analyzer couldn’t accurately operate.
[FromConceptToCircuit] was eventually able to make measurements throughout most of the 0.1-3 GHz range with a dynamic range of at least 10 dB, and expects a more directive RF bridge to give even better results. If you’d like to repeat the experiment, he’s made his Python program available on GitHub.
The ARRL used to have a requirement that any antenna advertised in their publications had to have real-world measurements accompanying it, to back up any claims of extravagant performance. I’m told that nowadays they will accept computer simulations instead, but it remains true that knowing what your antenna does rather than just thinking you know what it does gives you an advantage. I was reminded of this by a recent write-up in which the performance of a mylar sheet as a ground plane was tested at full power with a field strength meter, because about a decade ago I set out to characterise an antenna using real-world measurements and readily available equipment. I was in a sense field testing it, so of course the first step of the process was to find a field. A real one, with cows.
Walking Round And Round A Field In The Name Of Science
A very low-tech way to make field recordings.
The process I was intending to follow was simple enough. Set up the antenna in the middle of the field, have it transmit some RF, and measure the signal strength at points along a series of radial lines away from it I’d end up with a spreadsheet, from which I could make a radial plot that would I hoped, give me a diagram showing its performance. It’s a rough and ready methodology, but given a field and a sunny afternoon, not one that should be too difficult.
I was more interested in the process than the antenna, so I picked up my trusty HB9CV two-element 144MHz antenna that I’ve stood and pointed at the ISS many times to catch SSTV transmissions. It’s made from two phased half-wave radiators, but it can be seen as something similar to a two-element Yagi array. I ran a long mains lead oput to a plastic garden table with the HB9CV attached, and set up a Raspberry Pi whose clock would produce the RF.
My receiver would be an Android tablet with an RTL-SDR receiver. That’s pretty sensitive for this purpose, so my transmitter would have to be extremely low powered. Ideally I would want no significant RF to make it beyond the boundary of the field, so I gave the Pi a resistive attenuator network designed to give an output of around 0.03 mW, or 30 μW. A quick bit of code to send my callsign as CW periodically to satisfy my licence conditions, and I was off with the tablet and a pen and paper. Walking round the field in a polar grid wasn’t as easy as it might seem, but I had a very long tape measure to help me.
A Lot Of Work To Tell Me What I Already Knew
And lo! for I have proven an HB9CV to be directional!
I ended up with a page of figures, and then a spreadsheet which I’m amused to still find in the depths of my project folder. It contains a table of angles of incidence to the antenna versus metres from the antenna, and the data points are the figure in (uncalibrated) mV that the SDR gave me for the carrier at that point. The resulting polar plot shows the performace of the antenna at each angle, and unsurprisingly I proved to myself that a HB9CV is indeed a directional antenna.
My experiment was in itself not of much use other than to prove to myself I could characterise an antenna with extremely basic equipment. But then again it’s possible that in times past this might have been a much more difficult task, so knowing I can do it at all is an interesting conclusion.
